You can write NIFs in Rust (which is made even simpler by supporting libraries like Rustler). scrogson, for instance, is using this to fiddle with lower-overhead json.

Re: supervising external processes, an easy hack if you're writing the processes is to add a deadman's switch to both sides, and then launch the processes from a port in BEAM-land.

This effectively makes them supervised; kill the beam process and the external process will die, and whether they both get relaunched depends on the restart strategy the supervisor launched the child with.