Yeah, you're not going to get a much fatter tire in there - the fork and chainstays are narrow. Interestingly, the chainstays have plenty of room.
The Schwalbe Marathon comes in 700x25 or 700x28. The Conti Top Contact (fantastic tire, use them in 700x32 on my commuter) only goes down to 700x28, but that might fit (would be a spendy experiment if it didn't work).
Sticking with a 700x25/28 saves you buying new brakes.
You could also add a WTB dirt drop bar setup if you want a more upright riding position for the bumpier terrain. Dura Ace bar end shirters are readily available.
