Seems to work for git-remote-hg, anyway:
nix-repl> (builtins.fetchGit { url = "hg::https://www.public-software-group.org/mercurial/lfapi/"; rev = "34366bf575c8c77c8d3b76d32940c1658cb948a4"; }).outPath
"/nix/store/8dwyms22iwy4fq0b1593i34m88jk574j-source"
[0]: https://app.radicle.xyz/nodes/seed.radicle.xyz/rad:z3gqcJUoA...