After reading Pamela's queries about minimum safety requirements and regulations, I started looking at regulations in Montana to make sure I'm within them.
You don't need to register kayaks or canoes, as long as they're human-powered. Sailboats under 12' don't need registration either. I found, however, that by putting a sail on my 18' kayak, it becomes an 18' sailboat, and I need to pay the registration fee, and put on the ugly numbers. I double-checked this with the Montana Department of Fish, Wildlife, and Parks, who has the unenviable task of enforcing watercraft in the state. The local warden and head of the local state parks independently concurred and said that the 18' kayak-with-sail would be viewed as a sailboat.
I guess I'm not going to become a kayak sailor anytime soon. I can, however, use a parafoil kite, beach umbrella, or tent fly-and-two-paddle-halves, and not have to register as a sailboat.