I have used AO as a channel, but the setup is not a one click solution.
Have a look at this video:
- As a quick guide insert a “mix 8 colors” between the material node and your phong/blinn shader.
- Connect your phong to the base color of the mixer.
You have to insert the mixer in every material which is used by objects for ambient occlusion
- create a partition for all AO objects
- Get an override and add the color 1 property of a mixer node
- select the override, open the Rendertree
- connect a ColorStoreChannel to the override
- connect XSI AO Shader to the ColorStore
- open the ColorStore and add a channel called AO for example
Hope that helps
I think sometimes it is easier to setup an seperate pass for AO, but it depends on the scene and how many objects are invloved. For inserting the mixer a script would be handy.