Here's a possibly silly thought-

Would it be feasible to control the mcu from the host Mac via say zterm? That might make the LCD redundant, and allow an invisible "stealth" installation on the internal floppy connector. One would need to have zterm on the Mac already ...

or

have the FlopEmu default to the first image, and have a bootable system with zterm on there.

I note that you have a nice colour GLCD on your prototype. Is that just one you had handy? I was thinking the final could maybe use something cheaper, like those 1-bit mono Nokia knockoffs? Also suitably retro