Well in their defense, I did walk up and ask (I had already listed) about the JS at 13mins prior to the departure.

There were three agents there talking with no other customers, but they said sorry, flight closed 15 mins prior.
Probably true in this case since the flight not completely full. In the past I've walked up with 5 mins to go, but with peeps still in the Jetway, the agents usually will clear you.
I did not protest because 15mins is apparently their cutoff, but I just went over my the window and checked my rather bleak options for the rest of the day on my phone. One of the agents then left and went down the jetway to close the door. To my surprise, about 3 mins later she reappeared and said the crew saw me in the window and "authorized" her to put me on.
I was shocked, elated, and eternally grateful! We still got the door closed with 5 mins to go before departure time.

And BTW, I used to JS on Airtran occasionally and most of their agents would go the extra mile to get you on. One time I walked up to the gate as the door was closing, the agent had me write my name and employee# on a napkin as we walked to the door of the jet and said he was going to go back and add me after the door closed. Granted that was an "easier" time for everyone to JS, darn computers!