This isn't an answer to your actual question, however: I've been using postsrsd (https://github.com/roehling/postsrsd) successfully to forward email in a similar situation - users with addresses on my box that they want to be forwarded to a Gmail account. It has obvious downsides, but it did solve the problem in my specific case, where google was rejecting ~20% because of SPF failures.
-- Harald
