I do bulk posting with a little Perl tool called flickr_upload. Today, I tore into that code to see how it worked, and while doing so, added handling of some features that Flickr has added to their upload API. So it now handles overriding the set default "safety_level", "content_type", and "hidden" attributes of the uploading images.
I've sent my patch back to the maintainer registered in CPAN, but I've also put it online here.
What I had wanted to do was be able to override the default license. Flickr defaults images to "All rights reserved", but has the option of setting various Creative Commons licenses. Most of my photos on Flickr, I set to "Attribution-NonCommercial".