So the CK apparently has the ability to create new collision layers. There are also a couple of apparently unused collision layers that should (in theory) be modifiable.

Trying to use this functionality, though, results in three problems:

1. There does not appear to be any way to "point" bhkRigidBody nodes at new collision layers. NIFskope provides an enumerated list of (most but not all) existing collision layers, but the actual data appears to be random-ish numbers that bear no relation to the layer's FormID or any other value I can work out.
2. Modifying any existing layer or adding a new one marks all layers associated with it as changed in the ESP. I see why this happens, but I'm concerned about compatibility problems: if I create a custom layer that collides with statics, and someone else does, too, will statics still collide properly with both custom layers from both mods? Or will the last one in load order win?
3. Adding a new layer that changes existing layers also seems to randomly break existing collisions. While testing, I created a new layer that collided with L_TERRAIN, only to find my character now fell through the ground into the skybox in-game. This may have been something I did wrong, but I need to fiddle with it some more.

So I guess my questions are:

1. Can you use a newly created collision layer in a NIF? And if so, how?
2. How does the game handle multiple mods that modify the same collision layer?