still not entirely sure what you want to achieve.
however, it doesn't make sense to have a collisions matrix processed by jit.gen.
the dimensions of the gen matrix are equal to the number of bodies in the scene, however the number of collisions in the scene will vary every frame.
if you just want to create a matrix that has 0 or 1 depending on if a body is colliding with any other body, you could iterate the collisions dict and set the matrix cells each frame.
take a look at Max 6.1/examples/jitter-examples/render/physics/phys.world.collisions.js.maxpat