I appreciate that the syntax is ‘gone’ after compilation. I mixed up two points. My original question was about the concrete tooling needed to work on a mixed Reason/OCaml project.
My later comment was about something else: refmt does automatic conversion. So, there are two ways to go about it: One, have the user read Reason syntax, which may be better since it ties in with the documentation which will presumably be available only in Reason syntax. The other way would be to shield the user from Reason syntax completely by converting the source including mli files upon install. That way, when asking Merlin to jump to the definition, one would only ever see OCaml syntax.