Despite saying on libreboot and fedi earlier, also rboot, that 26.11 release and beyond are UEFI-only, I did add this today:
https://libreboot.org/docs/install/legacy.html
I no longer build GRUB/MT86+ in lbmk, but i wrote an easy guide for users to re-add it. FUN FACT:
The EFI version of SeaBIOS (as CSM) here is ReBIOS. I briefly used CSMWRap in 26.10, but ReBIOS is a continuation of it and it's... better in every way. It has full VGA emulation for starters, so legacy operating systems work - graphical mode switching works, and it can switch back to text mode. It's a full, libre VGA BIOS, unlike the SeaVGABIOS code. I'm thinking of how to port this to coreboot framebuffer (libgfxinit) for legacy SeaBIOS setups.
Anyway. here are some photos of ReactOS booting for the first time in Libreboot. Booted using ReBIOS CSM module, added to Libreboot EDK2 using the above guide. Yes, this is amazing:
(could also boot old Linux/BSD with this - e.g. OpenBSD from before it had kernel mode setting / drm support):