This should all be part of isohybrid now, but I have a vague recollection that one of the patches I sent upstream hadn't been merged yet - I'll check that. If you need it to be writeable then it's probably not worth most of this complexity, since it's mostly there in order to support booting off CD. Just generate a VFAT filesystem with an EFI bootloader under /EFI/BOOT and use syslinux to make it bootable on BIOS systems.

Using separate bootloaders is basically a requirement. We're using isolinux for the BIOS side and grub for the EFI side. Long-run, we can probably consolidate on grub2 for all of it.