I don't think the system you have drawn will take a lot of flex or sway. If you could triangulate the corners, maybe, but that adds a lot of complexity. The rain gutter is tremendously strong.
The Danish couple (Schmids?) who drove their 60 all over the world had a variation of this plan.
The Camel Trophy vehicles had racks joined to internal roll cages, not to roof rails (at least the Discos did, and probably the Range Rovers as well.) I think with all the room inside a 60 a good internal roll cage would be a tremendous mod. It's like a handball court inside there - plenty of room for some tube here and there. You could mount a killer rack system to it with no flex or sway, and it would also of course support the roof if you rolled. It would also form super internal lash points for harnesses, cargo, security gratings, etc. Would solve a multitude of problems.
Haven't seen it done though.