I've been looking for a solution for this for awhile. I use v-ray and they have a wire color output option so I've been using that in conjunction with the z depth for a bad but usable result for DOF effects in AE. I just can't seem to get the Alpha information to output on the geometry for z-depth / object id .. well any g-buffer really.
Sorry for no answer but at least your not alone and I hope someone knows what to do :D anyone??