I started working on 0ad in August, the commits I'm working with are dated to the beginning of September.
When you are not focusing 110% on one package it takes some time, especially when the distributed source code base is big. At first I was stuck on trying to unbundle nvidia-texture-tools, moved it on to a TODO task. Some days ago I started working on it again, and right now it seems as if fixing mozjs-38 is the only major obstacle between the current state and the point where you can play 0ad on Guix and GuixSD.
I hope someone else will take on packaging Xonotic.
Update: Thanks Clément!
Update 2: Packaging Xonotic wasn't as hard as I thought and was finished in early 2018 on one evening. Looked harder than it actually was in 2016.