Here is RJ's quote on the subject, from the Theoryland database:

Week 15 Question: When a channeler is forcibly turned to the Dark, is his/her former personality lost to eternity? Are they in a permanent state of mindless Compulsion? Furthermore, can a channeler forcibly turned to the Dark return to the Light unaided?

Robert Jordan Answers: They are not in a mindless state of Compulsion. Their former personality is twisted, the darker elements that everyone has to some degree elevated while what might be called the good elements are largely suppressed. I don't mean things like courage, which is useful even to villains, but they are unlikely to be very charitable, for example, and forget any altruistic impulses. Call it being turned into a mirror image of yourself in many ways. It is very unlikely that a channeler forcibly turned to the Shadow could find a way back to the Light unaided. For one reason, by virtue of the twisting he or she had undergone, it is very unlikely that he or she would have any desire to do so.
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Now, this seems a bit different from what we observed in Tarna and Mezar, so maybe it isn't the dreaded 13x13, though it does seem likely. Maybe the obvious manifestations of the change is why it isn't used often.

However, notice RJ says unaided. Fist off, he says it is unlikely, not impossible. Second, if aided, I think it could be possible. Especially if one had a talented healer about. Wit Nynaeve, Flinn, Samitsu, Sumeko, and Corele around, I still have hope for Tarna and Mexar.