Another crazy thought -- what about trying a newer Mac OS ROM (like the one that came with 8.6) with Mac OS 8.5? Does that still boot, and if so, does the floppy work? That might be a way to figure out if the ROM file contains the change.

It looks like the Mac OS ROM file starts out as an Open Firmware script and then contains an LZSS compressed ROM image that it decompresses. Decompressing the ROM image manually might allow us to look and see what all it contains...