The issue with AO is that if you render with no filtering, you'll need a very high samples count to have no grain. Tho you'd certainly have more control on falloff and spread than with a skylight. You can put a ambien/reflective Occlusion in the diffuse of the standard mat and put your diffuse in the "bright" channel. For…
I spoke to Uly at work today and his suggest was to render the AO in Mental ray, the lighting/diffuse in Scanline, and merge the 2 in photoshop. I'm keen on that idea if there is no other single-pass solution here.