Very nice antique quilts but I see your dillema. It will be very hard to find fabric that will blend in with the worn look of the other fabric. I think you may be on the right track of just covering those places with tulle.
But, if they are only going to be on display and the quilting is still strong, maybe they are stable enough. I just don't know that much about this type of work.
I would be interested to hear the outcome.