seabios is the project you're looking for, and yes now I want to go see if it supports being booted from GRUB:)

EDIT: The answer is yes and it's been done: https://g00se.org/2016/11/seabios-on-libreboot.html (granted, that's booting directly from coreboot to GRUB, but it should be the same booting UEFI to GRUB AFAIK)