short of the "time entering the line card",
the tsa imho, has absolutely no business in asking the traveling public to assist them in their security duties. 
i would have asked the "tsa person" for their i/d on the spot and recorded it as a uni doesn't cut it. if you are asked why, it's very simple...anyone can get a uniform but if they are in fact a tsa, and thus a gub'ment employee [save sfo as they are still private], they must by law show you their i/d. if they refuse, get a leo involved.
you could have also accepted the bag in question but when you approached the checkpoint, inform the screeners that a fellow tsa employee asked you to put this bag thru as a test.