It's not "unprofessional" if you make sure you don't act unprofessional about it. I've done this many times, it's just doing your job. Just make sure the antenna is pointed down if it's on the inside of the thigh, unless they request otherwise ( ahem). Depending upon a couple other things, if she has long thick hair that drapes down the back, it could hide the Tx while being attached to the bra or center back of the dress. This is where having a small Tx like SMv, or TRX900LT can come in handy.