So far, I seem to have had two unrelated problems that combined to confuse me:
- The Mac uses EFT only, not the BIOS that one would find on a normal Intel PC. This seems to be why Grub4Dos and the Fatdog64 installer don't appear to work right for me. They create a USB stick that would be bootable on a PC that has a legacy BIOS. Creating a Fatdog64 USB stick via "dd" gets around that particular problem, but the catch is that I can't, for example, make permanent modifications to the boot arguments, since the config file for the boot menu is on an iso9660 partition. Instead, when booting, I have to press 'e' and then append "savefile=direct:device:sdb3" to the "linux/vmlinuz" line at each boot.
- The loading of the savefile from the sdb3 partition of the USB stick wasn't working quite right. I seemed to be able to work around this, though, by adding "waitdev=10" before the "savefile=..." argument. [ETA: Actually, adding "waitdev=10" was enough to allow Fatdog64 to find the savefile.]