Have you pointed out the problem to the coach and asked for an "exemption"? For the life of me, I can't figure out what the motive is for the "no lines" rule anyway!? Not like a judgeb will deduct for panty lines if they can't actually see them. I say whatever makes the kid feel comfortable should be fine, as long as they dont show out of the leo. Thankfully our girls are allowed to wear the nude undergarments, and we use the GK ones. They also have to wear sports bras as they get older, and there is not much you can do to hide those, so what is the difference??
As for the seamstress idea. I had to have an expensive dress altered, and just looked in the phone book. Lots of choices, all small businesses or individuals. I asked if they had similar experience, and asked to see some examples of alterations first, and it turned out fine.