I know that the real issue here is that most Flickr users have absolutely no idea how to either find or change their licenses, however two pragmatic technical pointers:
1. The flickrapi gives you back the licence based on a simple query using the photo_id. I recently used this to double check licences using a custom Python script during a mass upload. For Commons, it would be not too hard to build friendly web-based simple tools to help users check licences and even mass change their own licences in their Flickr albums to be "Commons compatible" with a simple couple of button presses (using oauth). The well designed Upload Wizard can do the mass upload of their chosen albums to Commons.
2. I use a bookmarklet to quickly find images that can be used on Commons: javascript:(function(){if("".length==0){window.location="http://www.flickr.com/recent_activity.gne?days=1%22%7Delse%7Bwindow.location...)
Here is an example search for "cookies" using the bookmarklet: https://www.flickr.com/search/?q=cookies&l=commderiv&ss=2&ct=6&a...
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