Go to the user's page : www.youtube.com/user/<some user hash> and lauch the bookmarklet to open the URL of the corresponding feed !
Updated (20150804 to new the youtube API)
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                javascript:(function(){var%20l%20=%20document.location.href.replace(/user\/(.*)/,%20"feeds/videos.xml?user=\$1");%20window.open(l)})();
  
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                javascript:(function(){var%20l%20=%20document.location.href.replace(/user\/(.*)/,%20"feeds/videos.xml?user=\$1");%20window.open(l)})();