The Surly frames seem to be a good choice. Since it is not a conventional choice, asking if a 26" x FAT slick will fit in a 700c frame is probably not an easy question to answer.
The 29'r option is another thought, although I was trying to favor the use of 26" wheels. I have an assortment of spare 26"/mtb stuff that I would love to put into service.
Any other ideas/experiences? There are probably many ways to develop the "Urban Assault Vehicle" concept.