Pete doesn't read other webcomics, which is why it's extremely rare to see his involvement with anything, and Gunnerkrigg is being published by Dark Horse (and in England, so I don't think they were at the event) so they're not going to really want to put that in their daily comic. Dresden is on semi-hiatus as he works on his TV pilot. And Dominic Deegan's too involved with the Gary Stu to share the limelight with someone else...
Actually, I'm surprised they got PVP and Penny Arcade to chip into this.