Rubin's methodology of late seems to be "let the artist be the artist." He freed Johnny Cash from the horrible production of his 1980s records ("Silver"? UGH!) and yanked Neil Diamond away from the syrupy crap that he has used lately... Giving Neil his first best seller in decades.
I suspect that if he was to work with U2, his direction would be something along the lines of, "Be U2." IE he would not offer much in the way of direction. That doesn't seem to be what the band looks for. They basically treat producers as extra bandmembers, as far as I can tell, so they'd want someone who would offer them new ideas rather than treating them in a hands-off sort of way.