Thanks for asking, Iasoun! We discussed a bit more over PM, and I wanted to share the key takeaway here:
I’m happy for XoR fans to make and freely share fan-works in various media on a non-commercial basis. (I don’t believe in my ability to prevent it, even if I had any interest in restricting it…and I don’t, I consider it fair use).
I don’t want to approve any commercial spin-off products right now…both because I’d have to work through the implications for my rights to the IP of the XoR-verse, and because I’d want to have a role in quality control. Right now I don’t have time for either, as all my creative time is going into writing Game 2.
Since you want to make a free RimWorld mod that gives people the chance to play rebels in the Shayardene winter, knock yourself out! I might even pick up RimWorld so I can try it when you’re done. 