I love it! It is very similar to the design planned for the B-ski treehouse. Agreed with Matt; you gotta protect from wracking. It doesn't have to be at all verts and all sides, although that would be the sturdiest. It DOES have to be installed on at least 2 adjacent sides, tho. Simple stuff; plenty of ways to do it. I'll throw ya the link to my dock project for more ideas and inspiration .
Nice work, Eric. You're gettin' me nuts.