Well, from the top of my head, all you need to do is make a PCB which splits all pins from the original source to two destinations. The A500 ROM is a 40-pin DIP IIRC, and simply add a switch on the CS line.
A PCB can be made online with services like expresspcb.com etc.
Ok, I said all that just to wave and say "hi!". I'm in Sydney at the moment, just flew in from Brisbane after staying there for 5 days.
Heading downstairs to watch the game over an icy schooner. Later mate, cheers.
