So it would make sense to attach the railings to the tops of vertical uprights which doesn't make for a great connection.
You're on a good path with this plan, Rob. I think having some 1/4" thick connector plates (similar to Simpson connecting/mending plates, but much beefier)fabricated would add a nice visual touch if they were painted black, along with the exposed lag screw heads. If you are using 6 x 6 timbers, the plates would be something like 1/4" x 5" wide x 10" tall. Attach to the bottom half of the plate to the vert and the top half of the plate to the horz railing. Heck, if you can run the holes straight and perpendicular all the way thru the timbers, you could put identical matching plates at both sides of the railing structure and thru bolt them together. Paint black.
