In the good old days, you would ask for GLA-DEN on US in C and the agent would not find any availability because the domestic sector was in F. You would then ask for GLA-PHL in C, seat would be seen, and then you would ask for the connecting flight to be added on, and the agent would then tell you that the F rate applies for the whole hourney.
As far as I know, US domestic still books into "F". Have you tried asking specifically for the GLA-PHL C seat ?