I only skimmed through the post but I can tell it’s fascinating and I’ll love diving deeper into it.
A bit of a shameless plug - while not a dedicated Nix CTF - for a couple of years now I’ve been making sure we had at least one Nix challenge at every KalmarCTF. I haven’t been too good at cleaning up and uploading the challenge sources after the fact 
but still. So far every time we just have a firecracker VM per player who wants to attempt the challenge, and I guess because Nix is still niche enough we never saw any abuse or had the need for a more sophisticated solution.
That’s really neat! As mentioned in the blog post, I fiddled around with firecracker a little bit but decided to save innovation tokens by going with libvirt/qemu. For next year, we’re deffo going to use something like Firecracker (probably cloud-hypervisor).
Awesome. FWIW, we may be looking for more chal help next year (or possibly earlier, we may be trying it out at SCaLE) 
We got some interest this year, but it mostly had to sit on the sidelines because we wanted to prove it out on our network first, and wrangling DC wifi with a local everything was already complex enough. We have that figured out now (that, as well as more general event info, will be in a third blogpost, coming up).
Oh, and we still have to figure out what we’re doing with it in the off hours (like a free-play mode or something), but we have needed to do infra improvements before we commit to that.
Edit: in the meantime the NixCon CTF authors need help too: NixCon CTF Challenge Authors