It’s an excellent idea, and provides a bit of secondary protection if you’re not confident you’ve eliminated all the XSS vulnerabilities. But unfortunately the XHR workaround you provided renders it completely worthless.
It’s always nice to hear about obscure things like that (especially when written so eloquently), but that’s not a useful security measure. Even the 10 minutes it would take to implement it would be better spent checking your code for potential problems.