Instead of injecting something into the page, what about having the snapshotter running an is-it-done-yet callback periodically to see whether it returns true?

Instead of injecting something into the page, what about having the snapshotter running an is-it-done-yet callback periodically to see whether it returns true? That would allow some basic checks like "is this particular DOM element visible yet" or "how many entries are in this list" without needing to modify the production code in the page.