I can't imagine placing anything on Myspace that was not already published - therefore my licensing rights already limited to mechanical license. My permission is NOT required for mechanical license of a published work.
I understand the dangers of placing UNpublished works on Myspace - but I don't see that I have anything to loose with published works. ASCAP already tracks performance roylaties (radio play) for me - and my mechanical license agreements for those also publishing my sonsg are still in effect.