OK, I went ahead and wrote a flickr-to-commons upload tool myself...
WatchFlickr has an extra line per image, with a pre-filled image name suggestion and an upload button. This will launch the upload tool.
The tool will check for valid license again, get metadata from flickr, get categories from CommonSense, and upload it to commons as "User:File Upload Bot (Magnus Manske)".
You will see the progress, and the wiki text it uses as upload description. Currently, you will have to click one more link to go to the new image page.
I am not sure what happens if you try to overwrite an existing image. The tool should probably check and auto-rename the image. Something to do for tonight...
Yes, I realize the potential abuse. However, I see these as pretty limited, compared to "let anyone upload any image". It is probably easier to create a new user account and upload stuff through that than to * create account on flickr * upload stuff to flickr * mark it as CC-BY * run my new upload tool
I think the advantages (easy upload) outweight the risks by far. Worst case, block my bot...
Cheers, Magnus