The question was: :
"Would the lightbox work with the entire page, or just within the iframe?"
You may have answered the question, but not correctly. The lightbox javascript will only work within the iframe and not the entire page that contains the iframe. When a photo is displayed using the lightbox function within an iframe, only the background of the iframe is dimmed. The main page background remains unchanged.
Example:
Lightbox Iframe Test - MajorLeagueTalk.com