I don't know how to do this on AMD boxes but there are tools out there to adjust CPU power limits without undervolting, it uses the chip's built-in ability to throttle itself. I've had to do the reverse on a Dell which limits itself to 0.8GHz on any non-genuine™ power adapter, but in that case I limited the (Intel) CPU's PL2 (long-term power limit) to 25W rather than its spec 35W to save power. It thus keeps itself throttled below 25W consumption by the CPU.
I'm sorry I can't be more helpful with names of tools to try, but I just packed up all my gear to head across town, I'll try to come back and drop some URLs later. I have a little 5750GE box so I'm interested in whatever the solution turns out to be.