Anyway, hope this helps someone. You can download the AE (CS5) project file here.
If you want to figure out what I did, it's actually pretty simple, just a little painstaking to set up. Mostly it's about creating 2 duplicates of the same comp. In one you have the mirror, in the other you have a camera looking from the perspective of the mirror's reflection (This was the tricky part. I had to find an equation that finds the reflection angle. You can check my ReflectionExpressionTest comp for a simplified expression setup using the equation.) . The second comp is then projection mapped onto the mirror in the first comp, but because in AE you have to isolate the projection map, a third "projection map" comp is used.
There are a few drawbacks to this method:
1. You have to keep the two duplicate comps identical. So if you edit anything in one, you have to copy/paste your changes or pick-whip a lot of properties in the other.
2. Because there is a camera looking through the perspective of the mirror, the focal point of the camera is way behind the actual mirror surface. That means if there are objects behind the mirror but in front of the mirror's camera, you'll have to turn them off. You may even have to animate them on and off.
3. You can't have lots of mirrors reflecting each other. You might be able to set up a situation where one mirror reflects the reflection of another, but it's only one way and it gets really complicated.
Edited by jayfaker, 14 January 2011 - 03:46 PM.