OpenArena does support mirrors however they are limited.
For example:
You can't have two mirrors facing each other.
The other thing to take into consideration is, how Quake3 does mirroring, it replicates the entire visible part of the map and renders it. So if you have a seen without a mirror at 2,000tris. With a mirror it would be more like 4,000tris.
Just to take these points into consideration when using mirrors.