With steel I would say you've got no worries whasoever. But with aluminum, things are not so friendly. I do think that your frame is safe to ride however, because the downtube is under tension with no torque or rotation to speak of like a head tube or seat tube would have to bear. If you can't get TSA to make it right, you should be able to ride the frame without worrying too much about it. If you do take this route, definitely buff the paint off of that area to see if there is any fracturing or if it is a clean dent.

You could also get someone to weld a scab over the dent to ease your mind. Aluminum welds great with the right technique, and an oval patch about twice the diameter of the dent and bent to form smoothly around the tube would pretty much eliminate any worry about structural problems. Hell; if you found a good machinist, you could have them mill out the dented area and weld in a circular patch of sheet stock then grind the welds smooth and repaint. The only hint of a dent will be your frightening memories of it.