Great post and pictures. I made a much smaller version of this when doing a custom log peeled railing system. Posts 6-10" in diameter and only 8' long, with crossing timbers in the 6" diameter and less range. Built a plywood table with saddles to hold the posts or rails, then built a sled that my router would ride on. I drilled and chopped the mortises... didn't bother messing with a jig to get my Makita mortiser on there, but that would have been a good option for a production setup.

I semi scribed the round into the round - so my router template was a 4" diameter circle. I adjusted my router depth, routed out the circle and created the mortise. When I had the mate cut I would then insert the tenon and scribe the balance of the diameter by hand.

Can't find any photos of my table... lost them on an old PC. Here's a tiny shot of the rail installed.