First, you are right, the bitmasks are 32-bit. I have choosen 32 bit because this is the length used by Panda3D’s CollisionMasks, and because longer masks would mean less performance on 32-bit computers.
A Python callback is of course the most flexible method, but it is also the slowest (by far).
Now, since I know that one size never fits all I have made it possible to extend the collision filter callbacks. In you config settings you choose which collision filter algorithm you want. Currently we have two algorithms implemented. I can easily add another one (or two).
But YOU have to specify how this algorithm should work. I will read you proposal, and check if it is possible to implement and what the impact on the existing code is.