@Oliver - I've built an adapter based on the 'IBMkey25' design. However, I've had to use a PIC16F84 instead of the PIC16C84 originally specified. Although the 16F' replaces the 16C', there are a few differences.
Unfortunately, I'm no programming whizz and although I've hacked the code a bit to make it more compatible with the 16F' and MPASM, the adapter causes the Amiga to not boot and display a blank blue, green, red, or yellow screen. I've checked all the wiring, connections, etc., many times and all looks good. It might just be the hacked PIC definitions file that's causing the problem. Perhaps I'll have to try out another design, or maybe buy a ready made adapter (not as much fun though).