There may not have been much activity in this thread, but that doesn't mean that all work has stopped. Progress has been somewhat slower than it could be, but at least there is some movement.
I looked into the amplifier situation, and since Phreeze reminded us that one part in the TDA1308 line was being discontinued, the rest of them have also become non-stocking items, or soon will be. There are a number of other ICs that are pin-compatible and are still stocked items at Digi-Key (for now) but they are not necessarily drop-in replacements; we may need to adjust some of the values of the nearby passive components to make them work correctly for our application.
I have also found a problem with the instrument ROMs that causes playback to be of inferior quality (or just outright incorrect). I am investigating the source of the problem, and have a few theories, but haven't found the exact cause yet. Once it's fixed, I'll have to re-generate all of the test banks and start testing again.
The last bit of news for now is that I'm looking into the cost of production of our own PCB brackets. The drawing has been submitted to the engineering department at the manufacturer, so we should know soon enough if this is an idea worth pursuing, or would be so expensive as to be infeasible. So far, we're looking at 0.81mm-thick steel with nickel finish and stamped audio jack labels.