Is it possible to perform mirroring from a CMS 7 (source) to a CMS 6 R2 (target) site?
I don't think so but...
An export package from 6 R2 is compatible with CMS 7 as described here: http://world.episerver.com/Modules/Forum/Pages/thread.aspx?id=64767
Do a quick test if you can export the other way as well. If you can it's possible that mirroring might also work. New property types and use of blocks should be avoided I guess.