That is a clever idea. But the very idea of someone stealing a quilt and then entering it into a quilt show, well, that really does take a criminal mind!!! I'd at least go to the judges and show them pictures of the quilt in my house, or with my kids, etc. And make such a fuss that this person would be humiliated enough so she would not steal another quilt.
What about tags you can sew inside the quilt, sorta like microchips for dogs and cats? Ones that can be washed and still work? Or, even better, ones like the On-Star finders put on cars?
Seems to me I've read recently something like that for expensive purses and thousands of dollar boots. Was it here?