Treehouse booking is not open yet. I believe it will begin in February and SSR members will have a period of time to book before owners at other DVC resorts.
I believe SSR owners can begin making reservations on February 8.
THV opens June 1.
Not sure exactly when non-SSR owners can book, but it's about 2 weeks later.
I think it will depend on how much they publicize this new option. If it comes quietly then you may be able to luck into something. If there is a lot of publicity about it I think you'll have a harder time.