The short summary is: we currently have no way to boot off a disk on
an iMac or G3 desktop.

The problem is that the booting method that installboot(8) uses is not
supported *at all* by the new firmware.  The current firmware only
knows how to read an executable out of a file system -- and it only
knows about HFS, ISO 9660 and FAT.

In order to boot from a local disk on these machines, we need a way to
create a HFS or ISO 9660 file system (depending on whether it's a hard
disk or a CD-ROM) to put the boot program in.  We currently have no
utility to do this.