I ran a rough test for my own information and thought I'd share the results. I took a popular board 5-1/2' x 34-1/2" and sawed it in half. I then machined mortises for three 6mm dominoes evenly spaced across the 5-1/2" and centered on the thickness. The mortises on both sides were machined on the middle width setting, can't remember that actual width. I glued the joint with TiteBond II but did not get much if any glue on the dominos. I clamped it up and allowed it 24hrs to cure.
To load the joint I supported the board on 2 x 4s on each end then placed short pieces of 5" wide steel flat bar across joint as in the photo. The joint held 95 lbs and failed when I placed another 12 lb piece (I said it was a rough test). Since an evenly distributed load creates 1/2 the stress of a concentrated load (mine was actually distributed over 5" but close to concentrated) my guess this board could probably have supported 200 evenly distributed lbs and perhaps more if I had carefully applied glue to the mortises and dominos. The second photo shows the failed joint from the bottom side.

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