Comp wireframe over AO

Hi all,

I am looking for a way of taking the Viewport Render image and comping over a AO render. But in the compositor I want to mask out just white areas of the VP image leaving just the wireframe. So basically when I am done I want the wireframe over the AO.

I know that can be accomplished using Freestyle I am just seeing if it can be done in the compositor.

Thanks
Alex