The list of UEFI research OSes has grown by one. Justin Miller has created Popcorn:
popcorn: A toy microkernel x64 UEFI OS: popcorn is a hobby OS for x64 UEFI environments to play with building a microkenerl architecture. It’s far from finished, or even being usable – for now, it’s a sandbox for me to explore the UEFI architecture, microkernels, and OS-related concepts that I want to play with.
With Popcorn, that makes about 7 that I’ve seen (and I’m not watching academic news sources, where others may be hanging out). Here’s the other ones I’m aware of: