You are correct. Voice actors are not allowed to voice the characters they play in the real thing for fan productions and such. The one exception I've seen to this rule is Kyle Hebert using his DBZ announcer voice, but I think that's a loophole since it's not really a character. I have a feeling Dante will be forced to back out once he realizes this, lest he feel the wrath of the Viacom Copyright Ninja Brigade. This is assuming, of course, that he actually agreed to this in the first place.
Legal issues aside...I guess I don't really understand this. Then again, when it comes to fanfics, I've always been a very "by the book" kind of guy, so maybe I'm not the right one to ask. I admit, though, I enjoy that Watertribe AU comic by Rufftoon, so maybe this'll be entertaining, too. Who knows?