Yes, if you have a PHP server you create a PHP page to do a CORS request and as long as the responding server allows cross origin requests with the correct header you use that page to grab the information.
Then in your scene you would need to do an AJAX call to the PHP page that actually does the CORS request and then push that to the html2canvas object.
Ive done that method with the PHP CORS thing for web scrappers and other externally driven data stuff…
You can also take a look at this: Heres your CORS load... now make it work - Questions & Answers - HTML5 Game Devs Forum and see if you can make that work with html content.
Let me see if I can find my old PHP scripts for you.