long time rumor, but I have never seen confirmation of it. So, like much of L2, its just a rumor. There's been a post about this before, noone that posted there ever levelled a crystal to 11 or 12 in a rift.
The only change they made from doing this on Epic bosses that I am aware of is that only 1 crystal will level, not all 9 in the party. It appears also that there is no guarranty that even the one crystal will level. I know a Bishop that tried it in 7-8 attempts, he was the only one in the party leveling a crystal and it never did.
Wel lI have tried 10>11/11>12 in the commander rift. Tried 4-5 times with no luck. However someone in my alliance swears it works. Guess Im unlucky.
Search the forums for +rift +crystal +level. You'll find a few posts on this subject. All of them come to the same conclusion, noone has personally levelled or directly seen levelled a level 10->11 or 11->12 in a rift.