OK, I think I see the issue. If this BCOS (big chunk of steel) is installed in a location that puts part of the steel plate under the panel, and in such a way that a person could stand on its surface while working on the panel, then it might (I say again might) be acceptable (i.e., it might not be a violation of 110.26). If I came across this installation, and all I saw was the BCOS and the panel, I might not see a problem at all. However, if at the time I stopped by, there was a motor or other item bolted to one of the T-slots, and if that item were within 42 inches of the front face of the panel, I would call it a violation.
You may say in return that the motor or other item could be easily removed from its position, and would be removed before work would be allowed on the panel. I would reply that this would be similar to saying that you could place a file cabinet, or a desk, or a trash bin, or a shelving unit in front of a panel, with the claim that it could easily be moved before any work would take place on the panel. I would not accept that claim. We don’t want a future maintenance person to have to deal with the question of whether it is worth the effort to move something out of the way, when all they have to do is to one small and quick job inside the panel. The NEC does not allow storage within the working clearance space, even storage of easily movable stuff.
For my part, if I were the AHJ, I would not allow the BCOS to be located close to the panel unless one condition was met: No T-slots would be allowed within 42 inches, to prevent any item from ever being bolted in place within the working space.
