Well now you can! DEC Rentals has a huge Oak Treehouse with two available rooms which can be rented separately or together. Each room is 250L/week for 50 prims. Pay the rental box to rent then contact Marie Resch to get into the rezzing group.
The link will take you in front of a concrete platform with stairs. If the elevator is not there, touch the platform to summon it.
The first room is the one on the landing platform, and the other is up the stairs as high as you can go. Your area includes the communal platforms between the two rooms. You also have access to the entire half sim, which includes parkland, shopping, fishing, and a club.
Check it out!
http://slurl.com/secondlife/Old%20Hen/136/35/72