Using a baby grand, I think I'd use a StereoVibe preamp. That way both of the non-sustain piano pedals could control preamp switches- one for trem on/off, and the other for trem half-speed/full-speed. An effects loop on/off might be a decent alternative to trem half-speed/full-speed.
Given there's so much room to work with, I think I'd be tempted to do something other than just 4x12. Maybe a 2x10/2x12/2x15 arrangement or something like that, with a 200W power amp. I would think speaker holes could be cut right into the soundboard.
The first trick would be finding a baby grand that is small, cheap, looks good, and ideally is in bad enough mechanical shape to not feel guilty about gutting it. Maybe a piano in not-so-good cosmetic condition could be used if a body shop would be able to put a nice gloss paint job on it for a reasonable price.