I'm not sure why everything must go in ROM anyway, it seems very limiting if you aren't stuck with double-density floppies only.
Get the machine reading the drives, then load the rest from disk.
It's much more flexible and it's more easily fixed in the field, especially if CDRoms and USB are supported as boot media.
ROM usage for storing OS components was a good idea back in the times of slow and constrained space storage media, like floppies, early harddrives, and tapes (do you remeber tapes dont you?). As they were slow, rom was "the very best thing" they could use, and having portions of the OS over there, not only allowed faster operation, but they also enabled you to save precious storage space.
Times have changed: Rom is not that fast now compared to actual hardrives that are pretty quick and efficient. So a kickstart today, has really no sense to be in rom, it should be on a drive.
Only the bootstrap code should still be kept in rom.