Thank you all very much for your help and advice; the PR above has now been merged and Topiary made it into OCaml packages!
According to my CI, it builds with the system version of Rust on Alpine, Archlinux, Fedora and Ubuntu (last and LTS). Users of CentOS, Debian (all kinds), OracleLinux and openSUSE will sadly have to wait a little bit. (Unless they provide Rust themselves with eg. rustup.)
Now there seems to be a tiny delay between the PR being merged and the OPAM CLI showing Topiary. Also, I have it on good authority that the Topiary team is preparing a new release, so unless you’re extremely enthusiastic about Topiary, I suggest you wait a bit before trying!