Source: see 100 conventions. It's one of his big points of whine.
And no I have nothing against the man, he is one of the best Trek con guests I've heard and I would happily see him again. But "oh wah wah they didn't let me direct" is pretty ridiculous. If he'd already ruined his cred with TPTB by being late all the time (which is apparently the case, no idea if true) why should they give him even more responsibility than showing up and mouthing a few lines?
He also complains that he is/was a wonderful comedian and they never let him do comedy on VOY.