Better leave it alone. ONSTAR = OBD3.
If the tree huggers and the EPA ever get OBD3 past the lawyers, the emissions cops will be able to remotely disable vehicles deemed to be "gross polluters".
If OBD3 ever gets approved, when your ONSTAR-equipped vehicle develops an emissions-related fault code, you will get notification via email or the ONSTAR system that you have X number of key cycles to have the problem diagnosed and repaired or they will disable your vehicle. Sounds good, don't it.
Automakers have been fighting this one tooth and nail because of the liability issues the tree huggers seem to be blind to or ignorant of.
This was a big topic of discussion a few years ago when I was going to the VW tech school in ATL, and the BMW school instructors talked about it too (BMW was really sweating it, apparently). The EPA wanted to it implemented no later than '02 but the automaker's lawyers got it shelved for a few more years. All it'll take is some politician trying to make a name for him / herself and it'll be back again.
Basically since GM and Delco / Delphi paid for the development cost (so they could sell the technology to everybody else when OBD3 was implemented) and when it got shelved they re-engineered the system as ONSTAR in an attempt to make some of their money back.
Sorry to be so long-winded about it, but there you go. I'm not a conspiracy theorist, but we all better go put on our aluminum-foil helmets anyway, so the EPA can't read our minds.