I wrote about this a bit elsewhere, while it was in progress, but since I've used it for half a dozen pictures now - auto_cropr.py (which rather needs a new name) is working, for me at least. All it does is look on flickr at flickr.photos.recentlyUpdated photos within the last day, and tries to find a specific cropr_me tag and specific note-text (same.) Then it takes the rectangle of the note, makes a crop of the Original image that matches it, and uploads that as a new flickr image. It then links the note to the new image, cleans up the tags, updates the metadata on the new image...
There are a few holes right now - it doesn't copy permissions or safety (all of my stuff is public, so the defaults are right), it doesn't put new EXIF data in the cropped image (and it looks like libexiv2 supports working with in-memory images, but python-pyexiv2 doesn't yet - and I get the Original image directly from flickr, and never put it on disk...) nor does it handle rotation.
More relevant to having other people use it is that I've never worked my way through the parts of the flickr api that would let other people use the app. I'm using them properly for my own account, at least... if I do go that direction I should seriously consider using a flickr library that someone else maintains, too :-)