It wouldn't have been much of a problem were it not for fonts/fontconfig.
I took a pause on it yesterday, but I'll post what I come up with to the list in the next few days. It's been hours of research/strugging since I wasn't all that familiar with X11 fonts or fontconfig, nor am I all that familiar with the details of Guix. My solution is going to be a combination of a packaged font cache and a script to invoke `guix container`; ugly and rigid!
Hopefully Ludo or someone will be able to provide guidance on a better way, or come up with a general solution, including the option to either run in a container or a VM depending on isolation concerns. I'm not expecting Qubes-style isolation here, though. :)
(Typing this in IceCat running in the container right now.)