> Hardware emulator (like Super Gameboy) is 100% possible. Porting NES games to SNES is possible.
There's not enough time to upload NES video frames at 60fps like the Super Game Boy does (GB is 160x144@4 colors, and it's very close.)
So an NES hardware device like the Tristar basically has a separate video output cable. I think it may pipe audio through SNES pins (or more likely has that separate too), and really just has a BIOS that polls SNES gamepads and passes that through to the NES on a chip.
It may be possible to do some absolutely insane "dynamically generated" 65816 code that utilizes the similarities between the NES and SNES PPUs to pull off an NES emulator with the native SNES output path, but ... good luck. Entirely not worth the effort.