It's a very old, common trick in CG - especially for animation - going back to the early days of Disney.

Build your scene in groups that you in fact, use as layers. Simply hide and reveal groups/layers as needed to work on your scene to gain speed enhancments. When it comes time to render, you can turn on a single group/layer enable premultiplied alpha, and then complete the rendering. Obviously, you'll need to do multiple renders for each layer. However, this gives you incredible control over lighting and depth of field that can then be done in post render.

A great diary of how this was done with Carrara/raydream has been the very slooooooooooooow progress of Rustboy. I don't know if he is even using Carrara anymore for his renders but he did for the original sequences using the process that I described. You can check out his work at this link.

Mark