Actually.. I just hooked ub an A2000 keyboard to one of my A600 boards.
with a little help from these links.
Won't do much good for an A500, but luckily it's even easier to get an A2000 board going on the A500.
Compare the pinouts:
A2000:
Pin Function
1 KCLK
2 KDAT
3 n/c
4 GND
5 +5 Volts
A500:
Pin Function
1 CLOCK
2 DATA
3 RESET
4 +5V
5 N/C
6 GND