Glad to hear that one of two joints may be good! Would be interested in knowing more of your concerns on the dovetail. Here are some details:

The (irregular) beams are roughly triangular is cross-section, ~4 in. wide across the (slightly crowned) top and ~4 in. deep. The dovetail tenons have a nearly horizontal base that gently slopes upward (angle ~20 degr.) from bottom of beam (AT tree) toward the beam center (as you move away from the tree). Max. depth of cutout always less than one-fourth of beam depth; tenon length is fixed by the supporting block thickness---which holds the intricately shaped mortise---namely 2.5 in. The shear plane thus ends up being ~2.5 x 1.25 in., i.e. ~3 sq. in., giving ~7,000 lbf calc. max. shear load (which is 7x the max. anticipated load).

I plan to relieve the stress in the angular joint at the (non-tree, i.e. outward from tree) end of the tenon by cutting a gentle curve back down to the bottom of the beam. Probably ~3 in. long elliptical curve (not looking forward to this, as it will take a coping saw, and locust is really hard!).

(I had considered a dovetail with vertical sloping sides, but that would mean flipping the beams upside down, putting the broad face on the bottom. Not sure how this would affect beam bending, if at all, but it would give me no good surface on which to attach the decking.)

One other consideration makes me worry (much) less about the shear in the dovetail (which is, after all, going to be in tension to ~1000 lbf): The decking will form a screwed-into-all-beams continuous ring all around the tree. In other words, the deck itself will act to balance the tension loads on one side by the tension on the other, all the way around.

Thoughts?

David