If true, that (telling the volunteers to expect Crocker's Rules in the audience) seems at worst disingenuous and at best unwarranted. Taken literally, it translates to:

"I promise that the audience which will read your writings will consist entirely of people who don't get offended by anything you say, up to and including things almost universally considered to be directly and personally insulting." (Because that's what Crocker's Rules are, yes?)

And in general I don't think that "I have things to say, but I'm only going to say them to people who promise not to be offended by anything I say" is in the spirit of Crocker's Rules. I also don't think that it's a good attitude to take, period.

ETA (from http://wiki.lesswrong.com/wiki/Crocker's_rules):

Crocker emphasized, repeatedly, in Wikipedia discourse and elsewhere, that one could only adopt Crocker's rules to apply to oneself, and could not impose them on a debate or forum with participants who had not opted-in explicitly to these rules, nor use them to exclude any participant.