I think it depends on the airline, but most do allow it. I know UA does, and I believe that DL and US do as well. After the door is supposed to be closed, even revs may have a hard time getting in. Considering a nonrev shouldn't really be a cause for anything out of the ordinary (especially when it's under their control) I can see why a gate agent may not bend over too much. That said, I do believe in karma and do whatever I can to help my fellow nonrevs out.