I found this over on the AX84 board from just a couple of months ago, looks like someone was thinking along the same lines. I know nothing about mosfets though so I guess I'm going to have to read up to figure out if this could be implemented:
"A crude overkill sort of scenario I had in mind was to take a mosfet like a IRF820 (or a valve stage), power it from the same power cap as the most critical drive stages in the preamp, and bias it just at cutoff. Feed it from the signal somewhere, but don't take any output from it. Then at idle it does nothing but as the signal increases it amplifies half the wave and starts to draw current, causing the B+ to sag and the valve stage on the same supply capacitor to lose headroom. "
Is this something that would work?