I believe the reason hitch extensions are de-rated is because of the lateral force (side to side) that gets applied to the extension during use.
One way to alleviate the lateral force is to attach chains, etc. from the ball end back to the hitch at an angle to create a triangle. Research hitch extensions used for slide in campers and you will see what I mean.
You can have anything built by a good welder/ fabricator.
I made an offset extension for both my bike rack and hitch haul to clear my spare tire on my jeep.
Anything is possible.