Any kind of artifacts on still images when using HDcache are extremely rarely caused by the insufficient HDcache accuracy. They're a lot more often caused somewhere in the scene, mostly by poor modeling or some crazy materials.
About AO pass... BRDF AO seems pointless and will be soon removed. Official way to create AO pass is to use Corona Texmap element and plug Corona AO map into it... as Maru already pointed out ;)
Rendering AO pass to cover blotches in corners is a ridiculous way to solve that kind of problem. It is as if you had your shoe untied, and you would solve the problem by removing the lace from your shoe, killing a random person, then dissecting their body, tearing arteries out of it, then putting them on your shoes to replace laces and tying them up, instead of simply tying the untied lace ;)