Yeah, shadows and reflections might be a pain. Something else to throw at you, although I haven't tried it yet myself, is the distance G-buffer. If I'm reading the manual correctly (RDS, mind you), you should be able to create a distance mask with the g-buffer. Then, import the single render you've already made into AE. Place your current render in the AE composition twice (two layers). Put the distance mask in and create a transfer mode layer based on the distance mask. Use this to guide your gaussian blur, and your should be able to force depth of field that way. AE's gaussian blur is better than what I've seen in RDS or Carrara.... Just a thought.