Absolutely lovely application! I can't imagine a faster way of making joints and testing them.
I'm a complete noob to Newton, but I imported a custom wagon mesh and got the wheels working quite easily:
http://www.dr-code.org/hatter/playground_wagon.zip
("wagon.3ds" should go in the "customdata/convexhulls" directory. "wood01_d.tga" into textures, "wagon.xml" in xml).
But right now the wagon isn't all that useful as a wagon since the main body is a convex hull. Do you think in a future release, you might add the functionality of linking physics primitives to arbitrary meshes (or bones)? That would make the application far more useful for artists. Then I could approximate the wagon with a few boxes and be able to put stuff into it.
If you add skinned ragdolls too, please consider linking the physics primitives to the skeleton, not the mesh. Then a game engine could have a single skeleton for animations with a single physics configuration, but with multiple meshes for different characters. Makes life easier for games with multiples character meshes.
But that's just a personal wishlist.
It's a great programme already!