It seems his agent and he did approach a few comedians to get script help, but I guess they weren't succesful in this regard. For example, he did meet with Marc Maron to toss around ideas for the tour, as related by Maron in his WTF? podcast. Maron's primary idea was to center the tour around Sheen as a motivational speaker, which I for one would have found hilarious in a self aware kind of way. According to Maron, that idea was a non starter.
He didn't say, because he is apparently on some kind of zen-journey at the moment (and of course, he can't piss off people at his usual rate, since much of his podcast's draw is his ability to get big and controversial comedians to come chat with him) but unless I am illiterate in between-the lines-reading: despite blaming the cooperation not happening on himself for coming up with what he described as "non starter" ideas, I did get the distinct impression that Marc Maron was very concerned about Sheen's mental health and that he was secretly relieved that he didn't get the writing gig. Maron did not outright say so in any way, though. That is my surmission from how he ususally puts things and how incredibly careful he was describing this encounter.
But as to your suggestion, I believe they really did try to get some old pros behind this tour, and failed. For what reason, I know not, unless everyone approached reacted like Maron.
