I had another thought for your cart. Cut off the pipes that extend up the scupper holes, you don't need them!
Essentially you have already created a type of saddle with your sling idea. All you have to do now is weld two short pipes perpendicular to your frame like I have on my cart and then the boat won't tend to twist off, you'll also pick up some support. You'll need to run a web strap around the whole unit to keep it all together for transport. You'd be suprized how much the of the weight of the weight of the boat helps to keeps it down on the cart anyway.
If you still want to have removable posts, cut the posts off and make a upside down shaped notch in the shape of an "l" on the ends. Then weld a very short pin down inside your cross member frame so that when you insert the pipe down through the scupper holes and twist them, they'll lock into place. Get it?
I don't think threading the ends of the pipes and the frame is a very good idea because of sand, line up etc. etc.
Just some ideas I had while I was taking a shower.
Let me know what you think.