To celebrate my 100th upload, I threw together a little greasemonkey script that adds a "IQDB" button to the Danbooru upload page. Clicking it causes the image in question to be checked against IQDB in a new window or tab.
The IQDB side of things seems to work, but I haven't been able to test the Danbooru part because my upload limit has been reached. Once I fully test it, I will post it somewhere, unless the Danbooru or IQDB admins tell me they mind.