...I remember seeing a webpage years ago explaining how to locate the ROM disk in a Classic ROM dump, extract it to a disk image, modify it, then put it back again, giving you a rom dump with a custom disk image (it wasn't too large though, only a few hundred KB IIRC, enough for System 6 and a couple of apps).

I wish I could find this info again.

It'd be a quick way to giving a Classic a custom ROM disk (should you be able to replace the ROM chips with rewritable ones).