I think if you take this case to the point where someone is mounting a 5' bar to the hull pointing straight aft and trapezing off that, then it becomes part of the hull. A foot strap isn't giving you any more righting leverage than you had before and it's not changing the aerodynamics or hydrodynamics of the hull in a positive way. Obviously, when defining the outer dimensions of the hull the intent is to limit the waterline and available righting leverage (particularly when they start getting into the athwart ship dimensions of the rudder appendage). The footstrap doesn't increase any of these performance characteristics and is, largely, a safety device.

Now, you could argue that having a little extra ability to prevent peter-panning down the side of the hull can improve performance but foot straps are not prohibitively expensive and are easily available to all. People will be trapezing off the corner of their boat whether or not a foot strap is there - should they not increase their safety just a little? It just doesn't make sense to consider that as part of the hull in either case.