Researching work benches will often show many examples of the sliding deadman. I opted not to put one on my bench because I valued drawer space more.
On the rare occasion when I need to support a piece from the bottom I'll just use a clamp in expansion mode.
I find those squeeze clamps are such that when turned around in spread mode, the minimum clearance is like 6 or 8 inches because the jaws are so bulky, and they nearly never fit in where I need to, say, force a very tight mortise and tenoned frame apart.