That would be wonderful
The only problem I can see is that the collision box is a cuboid but the objects are made from triangular faces
The biggest problem I seemed to have with the custom objects was positioning the dummy collision boxes accurately enough to stop you from 'catching' your wheels on them. Maybe you could design some sort of 'snapping' to verts so you could butt 2 boxes up more easily, so the roof of the buildings on my map would've been smoother.
But yes, please let me know if you ever get round to looking at multiple collision boxes - it opens up great possibilities for custom maps