I find Houdini's Calculate Occlusion SOP slow and difficult to control. I came up with my own solution, a multi-pass SDF-based approach that offers better results, cooks faster and a higher degree of artistic control.

For anyone interested, I've uploaded it here: http://tmichael.info/FastOcclusion.hiplc [tmichael.info] …note that for my purposes, I didn't worry about generating a texture… but, since I'm sharing this now, I added a quick and very (very) dirty solution to extract the texture into a COP.