Some interesting updates:
- PD3 and PD4 are unused GPIO but connected to test pads. Could be used for hardware mods (but first those pads would have to be located on the PCB). I'll open one of the A320 this evening and have a look.
- PD22 is connected to the HOLD key (0=pressed, already pointed out in the comments by BouKiCHi).
- PB30 confirmed to be battery charge status input.
- PD20 is the battery charge control output (1=charge enabled). Actually, this turns on a NMOS which shunts a resistor to ground. This resistor is used to set the charge control, so in theory a PWM output could be used to achieve lower charge current. However, this pin's alternate function is SSI data output, so PWM might be achieved by spitting out continuously the appropriate data.
- The FM chip audio output is (as expected) connected to the LINE_IN inputs of the JZ4732.
- As I had guessed, the LCD read signal is not connected, and thus it is not possible to identify the LCD type or synchronize the GRAM refresh to the VSYNC.