From Goroh's Doc:
"Consideration
5c00-5fff has 3 uses.
Split Mode and ExGrafix Mode's VBlank is written so as to become
crowded, it writes a 0 and becomes crowded.
Every mode tries to go around ExRAM mode including reading but it
writes it, is effective in bulk and #5c-#5f is the output at times
where it is effective."

I can't really understand that in that broken english, but doesn't it mean that when the MMC5 is using those 2 modes that whenever something is written to 5C00-5FFF in VBLANK, it writes a 0 to it?

As for it works in emulators and not hardware. Hardly any emulators emulate the MMC5 well. The best one I found is Nestopia, which seems to play all the MMC5 game without glitches.......but I'm not sure and thats just my opinion.

Did you make an MMC5 devcart? I've been wanting to do that.

Quietust: "But oddly enough, one of the games I checked DID write to $5C00-$5FFF during rendering"

How many MMC5 games did you check?