So, would something like {{GFDL-self}} be used for this, or is a new tag needed? Does Commons have any guidelines on this or is tagging images you obviously didn't take yourself {{GFDL-self}} common practice? (cc'ing commons-l)

I'm not sure if that is the best tag, but at the same time I wouldn't yell at you for using {{XXX-self}}. The important thing is not so much that you, yourself, created the work, but that you, yourself, are entitled to release it under that license (and you hereby do so). Being the sole creator of the work is, of course, the most common case.

Dan