This dual foot switch is what I use for save/load states (also for MAME coin drops). You just map the switches to whatever key you like (F5 and F7 typically). It's the perfect solution, I think. I don't have to sacrifice controller buttons, can use original controllers, and the feet finally get in on the action!
I got mine to work by connecting it to my pc via bluetooth then setting it up with http://www.x360ce.com/ and configure all your controls using that. Then when I opened project64 I went to configure my controls there too and it works.