That's pretty much what I was thinking of doing. I have an old mirror I was going to practice on.
I re-did the mirrors last night with standard rainx - I couldn't get to the parts store for the anti fog - and it actually worked pretty good this morning.
Something I Do: In addition to the coating of Rain-X, I bought a bottle of Rain-X, >Window Cleaner<.. use it exclusively on my windows & mirrors (House Too). Maintains the Rain-X effect. No water Spots either.