Messing with U-Boot, in the Libreboot build system. Adding it as a payload on x86. I'm most interested in its UEFI implementation.
I have a working patch locally, for the 32-bit U-Boot build; now I'm doing the 64-bit one, which is what will be chosen on most boards.
I'm not making it a main payload for now. It'll be a secondary payload, selected from the GRUB(payload) menu.
This screenshot shows the QEMU target running U-Boot. Libreboot provides coreboot images for QEMU.