Unless the iMacs have a similar floppy driver at the Open Firmware level, I doubt they ever were able to boot from that internal floppy port. The floppy driver that I have found is in the Mac OS ROM file, which would only be loaded from an existing boot drive. So there's no real advantage to putting it in Mac OS ROM as opposed to an extension--at least that's my working assumption.

I dunno if it's even possible to fit a system small enough on one anyway