Reason is basically a syntax transformation from a JavaScript-like syntax to the classic OCaml syntax. Thus, they can sit on top of any existing OCaml compiler (e.g. ocamlc, ocamlopt, BuckleScript) without having to fork anything. The Reason team is working hard right now to build up a community around ReasonReact. On top of their upstream contributions, they are generating massive publicity and goodwill towards OCaml the language itself.
Cool. I didn't realize Reason was so loosely coupled from the underlying OCaml compiler.
We do contribute to upstream, and we've even hired a few renown OCamlers (can't say more atm, sorry). As a matter of fact I've kept a list of Reason's contributions, PRs or otherwise. I'll publish it in due time.