Made a bit of progress wiring up the off-board stuff. Still haven't drilled the panel, but this is complete enough that with some alligator clips I could test.
The amp sounds OK. A bit disappointing but OK. At higher gain settings it starts to clip in a solid-state not-so-pleasant way but that is avoidable by winding the gain or guitar volume back a bit. The weakest point is the speaker. It sounds like playing a guitar through a hifi system, which I am sure we have all tried at least once. I tested all three spare speakers (none of which are actual guitar speakers) and it made a big difference. Oddly, the best sounding speaker of the three is the old torn subwoofer. It fits into the cabinet so I will start with that but keep my eyes open for something better.

I am not sure it's going to work that well for the foot drum though, so I have decided to build the piezo preamp into a pedal enclosure instead. That way I can use it with any amp.