I want to make it so that when an admin sees a 'hidden' image it gets flagged differently, like with a different background color (red) to indicate 'Hey! This is DIFFERENT!'

This didn't work. What am I missing in my sleep deprived brain?
img.not_visible { border: 5px dashed #CC3333; }

Probably an CSS id does override that. Try something like #images.not_visible. #images is the id of the element surrounding the thumbs in the default theme (theme may use different css/html setups of course)

No luck. I'll fiddle with it some more (it may be my custom theme, but it's a hack of the default one, and I didn't change the css names)