Hi!
Using A500 LEDs for hdd/floppy activity monitor in a PC, cool idea, but it would be quite hard to mount it on your keyboard like "Ami style" unless you would put some additional wires..
and
It would be much easier to take the A1200 LEDs, I think- there are no chips on the 'LED mobo'. You just connect the wires and it works instantly..
Good luck with your experiment

send some pics of your work..