I never saw a piggyback cartridge at the time though, or one that was obviously unlicensed. Of course, this could have more to do with Nintendo's legal pressure than with technological possibility.
I think a lot of the early flashcarts were piggybacks. I was never into that scene though.
The main pirating route at the time (a device (oh which there were several) that sat under the N64 and had either an Iomega Zip drive or a CD drive for holding ROMs), had a port for inserting a real cartridge to use its lockout chip.