Wow Mark, I can't believe they wouldn't let you bring cameras inside. It seems to be no issue here in the missions. And when you can setup a tripod and catch the altar with 1 second of natural light, it is so rewarding.
BTZS teaches how to shoot a "black hole". It's a large-ish cardboard box spraypainted black inside with a shade like a lens shade over an opening cut in the front. You put this in your pictures, like a prop, and shoot away. The theory is it is pure black. No light that goes in comes out. So any density over base+fog on the negative you get... Is from flare.
I've heard you can get pretty much the same test by painting a coffee can black inside.