I've finally got hold of the step-up converter and connected everything together.  This won't merit an entry in the annals of neat wiring, but it does the job and everything fits in the case, even if it is only held togther with blu-tac right now.

"Did you say cut the red wire?"

Running the system has thrown up a few problems, but nothing too worrying.

• Amplifier resets Pi.  Turning on the amp after the pi causes the latter to reset.  I'd guess it's pulling a lot of current on start-up and dropping the power available.  Not a problem if the amp's switched on first.
• Noise.  There's a fair amount of noise coming from the pi, more than I noticed when I ran the DAC into my hi-fi amp.  I might need to add some flitering, or just shield things better.
• Crummy volume control.  It takes a fair amount of fiddling to get the volume right  I might have to actually buy a correctly specced pot instead of relying on a random salvaged part.

Crucially, the pi-ano has now been tested by a real pi-anist!