After many different attempts I found a way that works well for zdepth and ambient occlusion passes. This is what the beauty shot that combines diffuse, zdepth, and occlusion passes. It also has a little bit of color correction.
Here's the depth pass.
And here's the occlusion pass.
All of these renders are still using fairly low final gather settings, but I might render with higher settings depending on how much time I have to render.