If your custom shader implements custom lighting you can sample the screen space ambient occlusion value for your lighting calculations.
float ScreenSpaceAmbientOcclusion::Sample( float4 ScreenPosition )
Custom Ambient Occlusion
You can implement your own ambient occlusion solution simply by providing the screen space ambient occlusion texture globally from your own component:
commands.SetGlobal( "ScreenSpaceAmbientOcclusionTexture", AOTextureCurrent.ColorIndex );