Sonic Impact not only makes tiny little Tripath amps, but they also market NXT speakers (is it NXT? I think that's the name of the technology). The basic idea is that a flat panel is excited, and it's the vibrations in the panel that make the sounds.
In particular, Sonic Impact makes these speaker "pads" - just the electronics, which need to be stuck onto some flat panel (cardboard, a picture hanging on the wall etc.) to make any appreciable noise. They sell for about $20. I would imagine you could hook up these, and leave them sitting on a cushion (i.e. something that won't resonate) and you could burn in an amp without any appreciable volume at all.
Chad