The netmulticore library [1,2] worked well for me. The style there is to spawn worker processes and pass messages between them. I'm not sure how popular that library got but that style of programming seems to be pretty popular among OCaml users who need it. Many projects use something lighter-weight or just roll their own (e.g. Parallel in Jane Street's ocaml-core looks pretty good [3]).
I certainly wouldn't mind if you could swap out the garbage collector (a la Java - where you can choose between a fast serial garbage collector and a parallel garbage collector). However, I don't think it's the right tool for many tasks.