As I read the detail for enabling 2MB ChipRAM, the decision to re-use the JP7 pad and pin path onto the expansion connector may be your solution. It appears that is the address line enabling the additional 1MB of RAM (0010.0000-001F.FFFF). If it's disconnected from to the expansion card, the RAM on the card won't respond, and you will have only 1MB ChipRAM. The size of that memory is detected by Kickstart memory test routines upon early power on.
Assuming the above is correct, you might get away with a simple toggle switch on that line as long as the lines are short and it doesn't pick up any electronic noise. Otherwise, a bit of gate logic, connected to an actual switch, that allows or prevents the address line signal from leaving the A500->expansion would, in theory, do the same thing.