I've been "playing" with emacs-fj / fj.el for #guix a bit today and with the kind help of @civodul got it to work.
TIL:
* use latest git, the v0.6 shipped in guix is b0Rked
* in .git/config, use: url = git@codeberg.org:guix/guix.git
* (setq fj-host "https://codeberg.org"), (setq fj-user "janneke")
* although no documentation is shipped in guix, (info or otherwise), the website does have a README: https://codeberg.org/martianh/fj.el
Yes, that's right, the URL in .git/config must be just right and the "host" variable needs to be an URL.