I think search engines like Google should just standardize on a query string like your example. It always works, it's clearly opt-in, it's visible to the user, it's just as straightforward as a header (if not more) for web apps to process, and it's no more spoofable than a header anyway. If you think it's a bad idea to pollute URLs with such things, an alternative would be a URL fragment that is detected by JS.
Query string wouldn't always work, because the site may already use the query argument for something else; some sites may break or freak out when given unexpected arguments.
URL fragments aren't transmitted to the server, so that means you lose referrer analysis for static sites.
