Great post Adam. I agree that labor being disutilitous is not an empirical observation, but is contained in the formal construct of praxeology.
This is very closely linked to the discussion in this thread and this thread. Hulsmann points out that an actor must always value his end higher than he values the means to achieve that end. Mises and Rothbard apparently both failed to see this fact or its profound implications. He explains this mistake as a case of intellectual path-dependency from Bohm-Bawerk. I think we are looking at the same thing here.