Actually I have that exact gun. I can change my nipple to any of the three(musket, 209 or #11. Eventhough it has the bolt with ejector it is completely open and exposed to elements. So to answer your question, yes it is legal but not with the 209 nipple.
It is a great shooter too. I like it better than my TC Black Diamond. Mine has the 28" barrel and a Monte Carlo stock. I love it.