Note that Erlang gets away with this through being a virtual machine architecture; BEAM is about as write-once-run-anywhere as it gets, and the platform specifics are abstracted by the BEAM VM interpreter. You just aren't going to accomplish this with a native compiled language, without encoding as a virtual machine yourself (that is, the AST-based mechanisms).

Perhaps you should consider fleshing out ghc's current bytecode support to be a full VM? Or perhaps an interesting alternative would be a BEAM backend for ghc.