Wow, hate to revive an old thread, but I'm in the same boat and would like to hear what the OP did in the end.
Here is what I'm trying to do:
Ultimately, I want an app that could handle basic notifications from my carrier which come 1 or 2x a month, but allow all new outgoing messages to come from another app. Google Voice, in my situation, other people might want Hangouts or whatsapp or other apps to be their default for outgoing from the dialer/contact/call log screens. Problem is, in order for a app to be "default", it needs to actually have the handlers for SMS/MMS built in. I'm not saavy enough to write a while new app from scratch, was hoping to modify one enough to forward new messages using intent.