The NEC says that the clearance starts at the location of the exposed live parts, or at the front fact of the enclosure (if the exposed live parts are enclosed, as they would be inside a power panel). So tell us, what parts are live and exposed? I suspect that for any component that you mount in this rack, all energized parts would be located internal to the case. From the projects that I designed alongside an IT designer, I recall that each component receives its power by plugging into a standard receptacle outlet that is mounted on the rack. I can't think of any part of the rack itself or any thing that you would mount on the rack would have exposed live parts. In that were true, then perhaps the working clearance requirements in 110.26 might not apply at all. But the real bottom line is this: is there any maintenance or repair activity that would be done with the equipment energized, and for which the activity requires something to be opened such that a person's hand might touch a live metal part?