In the second episode of 'Royal Pains', Evan Lawson took a ballerina on a picnic at the beach in the Hamptons. He pointed out that Spielberg and Seinfeld both lived in the area. And in the pilot episode, we learned that Billy Joel lived within earshot of Tucker's family spread.
Even though none of them were seen, just the mention of their names (and locations) counts towards their League of Themselves tallies. It serves as proof of their existence in Toobworld, without the need for an actual sighting as verification. The series 'Movies Stars' was good for this type of embellishment on an actor's TV-life. Here are a few examples:
1] The Hardins lived next door to Tom Hanks, who holds anger management classes at his place.
2] The Hardin children were in a school car-pool with the children of Robin Williams and Arnold Schwarzenegger. (This was all pre-Governator.)
3] Apache Hardin lost Joe Pesci's dog while walking it.
4] Todd Hardin backed over Kevin Bacon with Reese's studio golf cart.