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you could add php code to the smarty theme.html, have it validate the page on a set basis (maybe once a day or week or month or just once, period). and display the image until a page doesn't validate.
that's assuming that it's possible to detect via php whether or not a page validated. i don't know the validation process, but i'm assuming you could have the script open a link to the validation script or server, and wait for the results, and store info in the database maybe, to let the script know whether or not the page is validated
or maybe better is just an admin-side module that could loop through all navigatable, local links on your site (similar to a search bot spidering your page), so that it only validates when an admin runs the validation script.
might be a good project for someone with time on their hands