Hi Neil,
That's not possible. The page has no way of knowing whether it is rendering valid xhtml. The validator link is as much a tool for finding invalid pages on your site as it is a badge for passing validation.
All the features included with mojoPortal were designed with the intention of producing valid markup, that is all the markup generated by the software is designed to render valid markup, however it does not prevent a user from entering invalid content into the database.
Taking the most basic example, the Html content feature is designed to render valid xhtml and the markup that it generates to wrap the user entered content is in fact valid, but you can easily make the user entered content invalid by pasting in stuff from Word (though there is a toolbar item to help you avoid bad markup from pasting from Word), or scraping some html fragment from another page (you might miss a closing tag or the scraped content may be invalid to begin with). Its also possible to add invalid content directly via html source view in the editor. The validator link can help you realize this has happened so you can try to clean up the content. The WYSWIG does a good job of creating valid markup other than things you paste in. It has the paste as text toolbar button and the paste from Word button to help with pasting so it doesn't add bad markup.
If you find a feature in mojoPortal that you think is producing invalid markup (not counting the user entered content) please let us know so we can fix it if its true.
Note that the link doesn't work on non-public pages because the w3c validator can't get the content from those because its not in a role with permission. To check validation on those pages you can view the rendered source of the page and paste it into the validator using its direct input method.
If most of your pages are validating and maybe you have a few pages with a third party feature that doesn't produce valid markup, I would not worry too much about it. The vast majority of people who visit your site never click the validator link and hardly anyone does it on all the pages. If you were really concerned about it you can make an alternate skin for a page you know is not valid and leave the link out.
Best,
Joe