I found the core script file, BAT format for Windows though, that I used to do exactly what you're looking for...

REM BootImg.Bat

@echo off
C:
cd "C:\Program Files\DOSBox-0.72"
if not exist bootimg.conf COPY dosbox.conf bootimg.conf
copy bootimg.conf temporary.conf /y
echo @echo off>>temporary.conf
echo cls>>temporary.conf
echo imgmount 0 %1 -t floppy -fs none>>temporary.conf
echo echo.>>temporary.conf
echo boot -l a>>temporary.conf
start dosbox.exe -noconsole -conf temporary.conf

REM BootImg.reg (Proprietary to Windows, to make a context menu item for IMG files)

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\WinImage\shell\Boot\command]
@="\"C:\\Program Files\\DOSBox-0.72\\BootImg.bat\" \"%1\""

REM Default dosbox.conf for DOSBox 0.72, might wanna use the default for your current version though

# This is the configurationfile for DOSBox 0.72.
# Lines starting with a # are commentlines.
# They are used to (briefly) document the effect of each option.

[sdl]
# fullscreen -- Start dosbox directly in fullscreen.
# fulldouble -- Use double buffering in fullscreen.
# fullresolution -- What resolution to use for fullscreen: original or fixed size (e.g. 1024x768).
# windowresolution -- Scale the window to this size IF the output device supports hardware scaling.
# output -- What to use for output: surface,overlay,opengl,openglnb,ddraw.
# autolock -- Mouse will automatically lock, if you click on the screen.
# sensitiviy -- Mouse sensitivity.
# waitonerror -- Wait before closing the console if dosbox has an error.
# priority -- Priority levels for dosbox: lowest,lower,normal,higher,highest,pause (when not focussed).
#             Second entry behind the comma is for when dosbox is not focused/minimized.
# mapperfile -- File used to load/save the key/event mappings from.
# usescancodes -- Avoid usage of symkeys, might not work on all operating systems.

fullscreen=false
fulldouble=false
fullresolution=original
windowresolution=original
output=surface
autolock=true
sensitivity=100
waitonerror=true
priority=higher,normal
mapperfile=mapper.txt
usescancodes=true

[dosbox]
# language -- Select another language file.
# memsize -- Amount of memory DOSBox has in megabytes.
# machine -- The type of machine tries to emulate:hercules,cga,tandy,pcjr,vga.
# captures -- Directory where things like wave,midi,screenshot get captured.

language=
machine=vga
captures=capture
memsize=16

[render]
# frameskip -- How many frames DOSBox skips before drawing one.
# aspect -- Do aspect correction, if your output method doesn't support scaling this can slow things down!.
# scaler -- Scaler used to enlarge/enhance low resolution modes.
#           Supported are none,normal2x,normal3x,advmame2x,advmame3x,hq2x,hq3x,
#                         2xsai,super2xsai,supereagle,advinterp2x,advinterp3x,
#                         tv2x,tv3x,rgb2x,rgb3x,scan2x,scan3x.
#           If forced is appended (like scaler=hq2x forced), the scaler will be used
#           even if the result might not be desired.

frameskip=0
aspect=false
scaler=normal2x

[cpu]
# core -- CPU Core used in emulation: normal,simple,dynamic,auto.
#         auto switches from normal to dynamic if appropriate.
# cycles -- Amount of instructions DOSBox tries to emulate each millisecond.
#           Setting this value too high results in sound dropouts and lags.
#           You can also let DOSBox guess the correct value by setting it to max.
#           The default setting (auto) switches to max if appropriate.
# cycleup   -- Amount of cycles to increase/decrease with keycombo.
# cycledown    Setting it lower than 100 will be a percentage.

core=auto
cycles=auto
cycleup=500
cycledown=20

[mixer]
# nosound -- Enable silent mode, sound is still emulated though.
# rate -- Mixer sample rate, setting any devices higher than this will
#         probably lower their sound quality.
# blocksize -- Mixer block size, larger blocks might help sound stuttering
#              but sound will also be more lagged.
# prebuffer -- How many milliseconds of data to keep on top of the blocksize.

nosound=false
rate=22050
blocksize=2048
prebuffer=10

[midi]
# mpu401      -- Type of MPU-401 to emulate: none, uart or intelligent.
# device      -- Device that will receive the MIDI data from MPU-401.
#                This can be default,alsa,oss,win32,coreaudio,none.
# config      -- Special configuration options for the device. In Windows put
#                the id of the device you want to use. See README for details.

mpu401=intelligent
device=default
config=

[sblaster]
# sbtype -- Type of sblaster to emulate:none,sb1,sb2,sbpro1,sbpro2,sb16.
# sbbase,irq,dma,hdma -- The IO/IRQ/DMA/High DMA address of the soundblaster.
# mixer -- Allow the soundblaster mixer to modify the DOSBox mixer.
# oplmode -- Type of OPL emulation: auto,cms,opl2,dualopl2,opl3.
#            On auto the mode is determined by sblaster type.
#            All OPL modes are 'Adlib', except for CMS.
# oplrate -- Sample rate of OPL music emulation.

sbtype=sb16
sbbase=220
irq=7
dma=1
hdma=5
mixer=true
oplmode=auto
oplrate=22050

[gus]
# gus -- Enable the Gravis Ultrasound emulation.
# gusbase,irq1,irq2,dma1,dma2 -- The IO/IRQ/DMA addresses of the 
#            Gravis Ultrasound. (Same IRQ's and DMA's are OK.)
# gusrate -- Sample rate of Ultrasound emulation.
# ultradir -- Path to Ultrasound directory.  In this directory
#             there should be a MIDI directory that contains
#             the patch files for GUS playback.  Patch sets used
#             with Timidity should work fine.

gus=true
gusrate=22050
gusbase=240
irq1=5
irq2=5
dma1=3
dma2=3
ultradir=C:\ULTRASND

[speaker]
# pcspeaker -- Enable PC-Speaker emulation.
# pcrate -- Sample rate of the PC-Speaker sound generation.
# tandy -- Enable Tandy Sound System emulation (off,on,auto).
#          For auto Tandysound emulation is present only if machine is set to tandy.
# tandyrate -- Sample rate of the Tandy 3-Voice generation.
# disney -- Enable Disney Sound Source emulation. Covox Voice Master and Speech Thing compatible.

pcspeaker=true
pcrate=22050
tandy=auto
tandyrate=22050
disney=true

[joystick]
# joysticktype -- Type of joystick to emulate: auto (default), none,
#                 2axis (supports two joysticks,
#                 4axis (supports one joystick, first joystick used),
#                 4axis_2 (supports one joystick, second joystick used),
#                 fcs (Thrustmaster), ch (CH Flightstick).
#                 none disables joystick emulation.
#                 auto chooses emulation depending on real joystick(s).
# timed -- enable timed intervals for axis. (false is old style behaviour).
# autofire -- continuously fires as long as you keep the button pressed.
# swap34 -- swap the 3rd and the 4th axis. can be useful for certain joysticks.
# buttonwrap -- enable button wrapping at the number of emulated buttons.

joysticktype=auto
timed=true
autofire=false
swap34=false
buttonwrap=true

[serial]
# serial1-4 -- set type of device connected to com port.
#              Can be disabled, dummy, modem, nullmodem, directserial.
#              Additional parameters must be in the same line in the form of
#              parameter:value. Parameter for all types is irq.
#              for directserial: realport (required), rxdelay (optional).
#              for modem: listenport (optional).
#              for nullmodem: server, rxdelay, txdelay, telnet, usedtr,
#                             transparent, port, inhsocket (all optional).
#              Example: serial1=modem listenport:5000

serial1=dummy
serial2=dummy
serial3=disabled
serial4=disabled

[dos]
# xms -- Enable XMS support.
# ems -- Enable EMS support.
# umb -- Enable UMB support.
# keyboardlayout -- Language code of the keyboard layout (or none).

xms=true
ems=true
umb=true
keyboardlayout=none

[ipx]
# ipx -- Enable ipx over UDP/IP emulation.

ipx=false

[autoexec]
# Lines in this section will be run at startup.

You don't have to. It was to show how to use custom arguments. You can load a different config for different games if you'd like, but for your first example it should probably be:

open -a /Location/Of/dosbox -c "imgmount a '/Users/io/Downloads/Prince.img' -t floppy" -c "a:" -c "dir /p"

For totally generic games that boot straight off the floppy disk, the default config tends to work just fine. Like I said in my other comment though, I used a temporary config file, based on the default config, but I would append the boot image filename/path to the [AUTOEXEC] section at the end of the temporary dosbox.conf file.

It might be easier for you for now to make individual config files for each game, but it should be possible like I said to make a generic script to boot basically any PC floppy disk image directly.

I'll see later whatever I have around.