So, the tests are in:
The good news is that this will totally work. The effect is believable, and this will allow us to take the shots we want and even tweak with the framing in post.
The better news is that the tests are all wrong.
I knew I would need to flip the image to compensate for the mirror that is supposed to be there, but I didn't think to flip the camera to the wrong side so that when we flipped it back it would be correct. And so, in both test shots, it looks like the cars in the background (the same car, actually) are about to ram the car from the side. So, on set, we need to make sure to put the camera on the other side of the vehicle.
So if you're ever going to do a special effect, even a really basic one like this, it is best to do tests to make sure everything is correct. Otherwise your totally "buy-able" effect will be ruined by unnatural physics.
We may shoot some more tests just to make sure we know what angle we need to use. If we do, I'll try to get some stills up here for comparison.
Your Media Production Mentor