Geez that's a lot of work for so little return. Although I admire your passion Soarse.
The mirror in Quake 3 is produced by a camera in the map which is projected to a user defined brush target. Similar to linking broken windows.
By memory I think it appears in the SH version of Radiant but not the original used for AA.
When following the tutorial, I found on creating mirrors for Quake 3, the names of the camera etc are slightly different but they are there.
I got distracted with other projects as the interest here seemed to have faded at the time. I'm not even sure if I still have that test map I was working on.
I found an MOH map with mirrors and PMed the link to Major but have since cleared out my PMs as it had reached the limit of 300 a few times.