Meredith, I love your idea of the pencil drawing or watercolor. That kind of gift will be long remembered, and the drawing can just pass with the house if it's inherited. Very nice.
But your friend signing her clients up for a subscription--not so good. They will get tons of junk mail from being on the subscription list. And probably end up on even more lists. Plus, if they don't renew, they will be bombarded with renewal notices for years. She should be careful giving out her buyer's new address. If she asks first, I guess that is fine, of course.
Suggest your idea to her. Her clients will love you for it.
BarbaraT, your clients can take their door knocker with them if they move.