There is one day a week that photography is allowed in the Abbey. I believe its Weds. but it may have changed, and its after evening prayer.
When I was there, I arrived too early, and since I didnt want to sit through prayer, I walked around the cloisters and accidentally founc myself in the private areas reserved for the employees and priests, and it was awesome.
the visitation after mass was very cool too, it was less crowded and the lighting was almost eerie.
I totally recommend going at night.