A thought occurred to me as I was writing a reply to my post just now, and its more important, so...
My suggestion would be to have a similar setup, but instead of it being one mirror (and in a garage, no less), you would have a laser at one end, and a series of mirrors set at intervals that reflect towards the camera sensor (all within a vacuum). The mirrors cut the light (similar to how a Schlieren system works), and are arranged in such a way that the light beam is divided more and more by each mirror.
The goal would be to see if each mirror reflects the laser beam simultaneously, or if it reflects it in sequence, and if so, how long does it take for the light to go from laser to the first mirror versus from laser to the next mirror. The camera could be repositioned towards or away from the laser side to see if it makes a difference. You could also test different distances between camera and the row of mirrors.
Sorry! Forgot to ping you, @Clete