All they would have to do is give you a phone number to call (same as if you called for them to unlock the doors remotely) and not insert themselves into anything to do with the billing of your cell phone account.The Bluetooth should be just that...an interface device simply to allow sending/receiving calls by voice, and no more.
So the nav features via Onstar will remain as is, and the Bluetooth will be--like essentially every other vehicle currently with it--that will be to simply integrate calls. Interconnecting your specific phone plan minutes (which can be through a variety of companies with different policies) into working certain features isn't something used anywhere now, and would be quite complex to sort out for GM.
Onstar will remain with the built-in services for a fee it has now, and Bluetooth will simply allow you to connect your own phone for calls and not only be able to use the Onstar/Verizon number.
It will be interesting to see how this develops, and specifically what models it is available on and how it is integrated.