Pretty neat idea, Jim! That may solve a point of indecision that I had a while back. When I replaced the stock steel wheels with Fiat Roosevelts, I didn't have any caps for them, so I improvised with these mirrors (stock steel wheel with hubcap on the left, and Fiat Roosevelt with a mirror center cap on the right):
I eventually acquired the correct Fiat Roosevelt center caps shown here, but I always sorta liked the mirrors, and wished I had an easy way to swap them back and forth like your magnetic inserts:
