I'm very concerned with compatibility. You can't guarantee that coreboot and payload match. And in case of mismatch you get a memory corruption that is very hard to trace. Can we please change signature of cbmem entry?

You mentioned having trouble building GRUB. Can you detail those?

What do you mean by not having hardware supported by grub-coreboot? Grub-coreboot should work on all coreboot-supported boards.