It's hard to imagine what Bono would do at Bruce's show. The "special guest" events so far on the Springsteen tour have been very low key -- Nils Lofgren, for example, just sort of standing in the background playing a very low key solo on one song. I think in its current incarnation, the Vertigo tour could handle a significant guest appearance like Bruce or someone else major without much of a problem. But, having seen both tours so far, it's tough to imagine how anything but a token Bono appearance could work at a Bruce show.
That said, the one exception might be in New Jersey. Even during his prior serious accoustic tour, Bruce opened it up somewhat in New Jersey -- allowing himself a short playful period during the otherwise "an evening with" show. I guess barring that, I'd be extremely surprised to see Bono show up at a Bruce show to do anything other than strum guitar and sing some backup lyrics on one song, which is hardly worth it.