Is there a way to produce and export an ambient occlusion map in Zbrush? (maybe through zmapper)?
perhaps baking a material to a texture via this thread/script:
http://www.zbrushcentral.com/zbc/showthread.php?t=49966
(I haven’t gotten the combined texture to work just yet, but
it’s probably user-error on my part)
hope it’s of some help. 
WailingMonkey
Use ZMapper to render out a cavity map, then use that as your AO map.