If the HLDA signal/pin is potentially being cannibalized for this project... and if it is the last I/O that could possibly be used for something... I wonder it would not be a good idea to implement a
1-Wire interface on this pin. Then multiple (255?) devices could be addressed by signals on this pin. For example something like this
DS2408 could be used to provide parallel I/O. Perhaps to control a sound generator. There are probably better chips than the 2408 but I couldn't find one real quick. There should be something like a shift register that is fed by 1-Wire protocol. It'd be really cool if this could be used to simlulate (emulate?) a 88_4PIO.
No real idea how hard a 1-Wire interface is to implement. Many microcontrollers have 1-Wire in hardware for a couple pins. No idea about this pin on this microcontroller. I imagine it is a lot of work for Mike in the firmware. But just dreamin' and figured I'd toss it out there.