I think they really need to trade him now for salary relief. The Sox have already folded the season (trading Peavy, Rios, Thornton, and Crain) and the fans have tuned out, not that Dunn brings any fans to the park anyway. The money they'd save by dealing him now compared to dealing him at the deadline next summer is considerable.
As far as who will DH in his place, it's pretty easy to find a DH (or a poor-fielding 1B) on the scrap heap. Production won't matter until the cornerstones of the rebuild are in place. They could run Travis Ishikawa out there for all I care -- the goal during a rebuild is not to win games.