Since Reason compiles to native Ocaml binaries I'm assuming there would be no problems using it with MirageOS? Google doesn't bring up any answers other than an older HN comment[1] suggesting support will come soon.
After, it's about a convenience easy way to compile your Reason code with an OCaml code. Reason is just a preprocessing, you should use the `-pp` option with /your favorite build system in OCaml/ and, I think, it's enough.
Thanks for the answer; the other comment alluding to support being recent and the lack of information had me a bit worried. I guess the lack of information is because it just works ;)