I was going to edit my response to add that you could just fab up an lunette insert as the actual hitch on the Gooseneck is adjustable, and replaceable. In fact I was looking through these catalogs at some of the optional inserts.
I myself want a king pin option so I can haul my GN with my M915. But I have this feeling I've seen the lunette ring option in the past. Unfortunately I haven't found it.
I've seen quite a few people put super hitches on their M35's. With one of those you could easily add the ball and drop the GN on that...
I realize your solution was one to combine efforts, however I personally don't like the idea of another protrusion to worry about, either in my bed or finding it with my head...
Sometimes there is no perfect solution.
I had thought about that possible negative consequence also..... "That's gonna leave a mark."