How did you get around the restriction they put in on only being able to search for a single MD5? just spamming the server with requests? I once made an offline image tagger (it's glorious days are over, nobody gave a shit about it and I was sick paying for the server for hosting it) and made a plugin for it to grab the tags from e621:
http://i.imgur.com/36qdSf6.png
but ever since they removed that feature and only allowed searches for single tags, it kinda just lays around on my hard drive, I'm too lazy to tag everything manually. If searching for multiple md5 tags works some other way, I'd probably use it again :)
If you're just spamming server requests, then that's not working for me, a proper pause between requests would make it have ages and not putting in a pause would not sit well with me, I don't want to punish the poor server giving me the infos :)

Also, out of curiosity, where do you put the tags? In the exif data? PNG does not have exif but metadata chunks, but gif have none, so can they not be tagged? or do you put it in an alternate data stream? I toyed with that idea, but it only works on NTFS file systems and all the tags are lost when you copy/move the images to any other, like to a USB stick. Or does it use an internal database so the tags are only visible when you open your program? (that's the way I went back then with my tool)

EDIT: and why did e621 deem it neccessary to remove all spaces after my punctuation marks? lol
EDIT2: and here they are again. what.