Cool technique and an interesting problem. I've never done it but I can imagine tying two butterfly knots at the center of the rope, separate them by a little less then the diameter of the limb, put them under the limb, run both ends of the rope over the top of the limb in opposite directions, drop the ends down through the loops on the opposite butterflies and let the ends run to the ground (on two sides of the tree).

This uses two knots not one of course. Each leg of the rope is independently locked if one side of the rope fails. You could also put two or three wraps around the limb before putting the ends through the butterfly loops. Also could add a screw_link_ on each butterfly bight and run the rope ends through those if you don't like rope on rope for an SRT anchor.

It could be done with a clove hitch, but that would be dubious security if the rope failed on one leg.
-moss