But bit 0 is used for standard controller input on the NES.
As to hacking the ROM (building the controller, well I'm going to assume you know more about that), it's probably very simple if you find the controller input routine (and by finding, I mean use a debugger to see a breakpoint for reads on 4016/4017 and you'll find it instantly) to switch it from bit 1 to bit 0. Games usually just read $4016/4017 then lsr/ror the bit into the Carry bit then ror/rol the Carry bit into some RAM address that stores the push/hold status for the entire controller.
So, NOP one of those shifts that moves bit 1 to carry through bit 0. (and probably NOP the part that stores the original bit 0 data)
A normal controller will no longer function correctly, but there's probably no way to make it not function at all.