Things have changed since I last posted about this thread about MTB knobbies or slicks on the Pittsburgh to DC trails. The general consensus is that 26" semi slicks would do fine on the crushed limestone and gravel.
The mountain bike lost an argument with a car, and I picked up a Novara Randonee with 700/32 tires. I'm thinking that the differenc in tire width will change matters a little bit.
So: Cross tires with a mild tread? Something along these lines: