I have not read these articles that mentioned, but I am assuming that they are a version of snap-line square-rule. This is basically the same layout for hewing a log square except, instead of the cross section and snap-lines marking the edges of the timber one is marking the center lines.
These center lines are also the plumb lines. This allows one to use tapered round posts as all measurements will be based off these center/plumb lines.
As for the square timbers that intersect the round posts, keep the ends square/plumb and house them into the round posts. The hard part will be keeping the back of the housings plumb. This can be done with a router equipped with an extended base (24-30"x8-12" piece of plywood reinforced with batten strips), which will ride along two boards wider then the radius of the round timber at that point of the housing. The edge of these boards should run along the center/plumb lines. This setup allows the router base to "hover" over the round timber. One may need to shim these boards to keep them square to each other and not tilt in on the round logs. These "batter boards?" are similar to the box, but are adjustable for different log thicknesses.
I am not sure if this is a good explanation, and I am sure there will be questions, but hopefully it helps.
