It sounds highly unlikely, but I'd absolutely love it. Maybe a more realistic possibility is Billie and Norah opening for Green Day at some point, a-la Foxoro/The Network.
I think even more likely than that would be Billie Joe making a surprise guest appearance at one of HER shows. It wouldn't confuse the genre/mood of her intimate concerts as much as Foreverly imposed on a raging punk crowd Plus, Norah is touring right now I believe, and Billie's the one on "vacation". I think it would make more sense for him to pop up at one of her shows at some point than the other way around.
But they're both very busy.