I discovered this by accident.
If you add Sonic & Knuckles to your ROM list, it will recognize the next ROM in the list as a "locked on" cartridge. It makes sense, since that's probably what resides in the address space past the S&K ROM image.
This can be a bit bothersome, though. This means that S&K needs to be the last ROM on the cart to function normally. It's also pretty cool. I wonder if this works for other cartridges that had "lock on" technology? No time for me to test this now, though.