After looking at the boot situation for GRUB in regards to UEFI/EFI, any support of a GRUB/(U)EFI boot method is going to be complicated.

As a simpler alternative, it may be best to integrate syslinux support into the next version of boot-update and use this to boot UEFI systems. More testing is needed to see how well it handles such scenarios, particularly on wacky systems that normally won't boot GPT partitions without UEFI FAT32 (like my amazing ThinkPad X220T)