With OgreNewt, I don't believe it is possible to disable physics, and only enable colision detection, maybe we could have an Actor class, then a PhysicsBody class, and a PhysicalActor class. Then the PhysicalActor would inherit from Actor and PhysicsBody. If the user wanted to, then they could make their own class inherit from Actor, so they could do whatever they wanted it to do. Such as adding methods for translation, rotation, etc. by using Ogre methods, and not have them controlled by physics.
As for the Rigid Body Designer, how far do you plan of extending it? I was thinking of adding support for joints, and making it a bit easier to use. I think a in-engine gui would be plenty for that. But, if you are going to take it farther than that, then it would be good to have a native gui. The only problem I could foresee, would be if you use WGE to program the editor, I don't think Squirrel has wxWidgets bindings. Of course you could always use WGE from C++.