On Physics Update...
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TL;DR
Just expose everything from Chipmunk2D and call it a day. Head out for an early lunch.
I mean it. EVERYTHING.
Including the properties and methods in the parent class of joints and bodies.
Physics engines are already a relatively high abstraction.
Chipmunk2D is a better abstraction, in many ways, than Box2D.
But Chipmunk2D is also more performant and offers other advantages over Box2D and 2D versions of Bullet Physics.
Chipmunk and cocos2D's rendering speed (once game loop and timing issues fixed) are the headlining features of hyperPad.
Providing Chipmunk's benefits to your users will empower them to do their best work, and make your app more popular, cool, famous and desirable.
Doing things this way is the least amount of work, and requires zero consideration on your part. Simply surface all the properties and methods, and call it a day.
Spend any extra time on improving the Physics Editor which, quite frankly, has some low hanging fruit in need of picking. Grids and snaps in the scene units , multi-vertex selection and editing, boolean merging of shapes, and grouping of shapes.
Physics editor should also be a live working space for creating, setting and testing joints and other properties. A physics "playground" for lack of a better term. This feature, alone, will see you do well in all markets. Not least of which will be educators wanting to use their iPads to teach maths, physics AND coding.
One more thing...
...add booleans and Undo, to the "behaviour" editor.