This is a great idea and I just submitted it on the DNA feedback page. We all pay the same fee and paid the same for the DNA test but the private tree users have a HUGE advantage over public tree users. The same rule should apply: If their tree is private but a potential match is public, the private tree user should get the same information (or rather, lack of it!) that the public tree users get. In other words, they can't see the public tree unless they send a message. I also like the idea of a surname match.
By the way, smsitton and I are 7th cousins once removed. Public trees. :-)