I could be wrong, but i don't think an SG is going to have enough room on board for the battery and preamp electronics, unless you route out a very large second control cavity.
If you do go for it, run stereo out, and send your piezo signal to its own
acoustic amp or PA. Piezos sound like crap through guitar amps.
For on-board switching, you probably want to have a mag/both/piezo mini-switch and either a blend pot or a separate volume control for the piezo signal.