Right now, I'm not iterating over all iso's. I'm only using the guix-install iso directly.

As you mentioned the 3s, I just noticed, the uuid is simply the 1970-01-01-19-41-16-79 string (from the search command in grub.cfg) with 3s in front of all other digits.
So very deterministic indeed.

However, if I run `probe -u (loop)` in grub, I get 1970-01-01-19-41-16-79 as uuid, not 31393730-3031-3031-3139-343131363739.