or create a photoshop action and do it just once in Photoshop. After that just batch script stuff inside Photoshop (file > automate > batch processing or something like that once you have your script recorded).
I guess you could do that with scripting or do in Photoshop manually. Just create new black bitmap that uses as alpha channel the inverted AO render (white = key). But honestly solid RGB layers are pretty common compositors, like reflection, light and or shadow maps they are usually used with blend modes in AE, PS or…