Yes, I tried one - I didn't make it clear in my first post. Bouncing the light from the original source off a red mirror does nothing worthwhile and it was pretty obvious. As others mentioned, you inevitably lose some lumens in the process.

I was just playing around with the mirror in my back yard, but I saw no reason to bother working with it any more. Offhand, there are probably about forty or fifty more effective ways of signaling at night (well, ten or twenty for sure).