I've encountered this dilemma several times in museums and libraries and it's always worked out; sometimes you use the gain in the camera. other times you light very carefully to approved light levels (they read you lighting with a footcandle meter). Nowadays with slow shutter speeds and variable frame rate cameras (like the Varicam) you can shoots stills (or night exteriors for that matter) or any other non-moving subject at slow frame rates and shoot almost in the pitched black. Does your camera do that? If not beg, borrow or steal the appropraite unit.