Using physx for something like this isn't going to work very well. In those games, you were dealing with "pixels" being on or off.
You can approximate the pixels using a grid of spheres, but you'll need custom drawing code to generate your polygons. Again, you'll be limited in how many spheres you can have - make sure they all are set to not collide with anything else. It is likely to be easier to roll your own physics - basically a 2d bitmap of the world, and collide stuff by raycasting through this.
A good reference for a game like this done in a GPU accelerated engine is Disneys "wheres my water" series on iOS.