I was able to sucessfully run the MIC from the Pioneer BTB200 to the Onstar MIC location on the ceiling.
The outer edge of the Onstar MIC lifted up just a bit with a flat head screwdriver (It didn't pop out all the way easily, so I didn't pull any harder.)
With the Driver A-Frame molding removed, it wasn't too difficult to use a fish tape to get a pull-string from the Left upper corner of the A-frame to the Onstar MIC location.
I now have the Onstar MIC in place, and the Pioneer BTB200 MIC wire exits at the edge of the Onstar MIC, with the Pioneer MIC Velcro'd to the ceiling at that location.
Unfortunately, the Bluetooth unit never powered up, so I am waiting for a replacement from Crutchfield and can't report on the accoustics/sound of the MIC in this location.