I don't know if this is what your looking for and I haven't researched this response well. There were some homemade rom switchers on Aminet that were dirt cheap.
If I recall, the two roms were piggy-backed (pin 1 to 1, 2 to 2, etc) except the pin carrying ENABLE or SELECT. A simple toggle switch would ground this pin alternately on one chip but not the other. Simple!
If you find and study these docs, it should provide insight to modifying your switcher. All of the extra electronics on commercial switchers were for watching for an unusual signal (like reset being held for 3 seconds) and sounding a beep to indicate a rom switch.
EDIT: I suppose, if you figure out what the signal transition on the left mouse button is supposed to be, you could simply add a pushbutton that does the same.
If you are handy with a soldering iron and understand the material, this should not be too tough.