Yikes, there are a lot of threads in the features requests board!
Since this is the most recent to request batch-adding keywords, let's use it for gathering ideas on how exactly this will work. I've been working on such a system for my own gallery (new baby & lots of family interest) and have been thinking about the interface a lot. What's the most complete and also simplest way to add keywords to lots of photos? And where do those keywords come from (previous keywords, new ones typed in, etc)?
From a few searches, I've come up these threads that are relevant:
Gathering keywords from other fields -
thread - I'm not sure why you would do this besides not having the "title", "album" fields on the search form. One use I could see if gathering keywords from the filenames if you use a specific regular naming scheme that for example always puts the place in brackets at the end. A regular expression or other pattern-matching operation would have to be done.
Entering keywords on the thumbnails page -
thread - Makes sense to me - similar to the "
Add Thumbnails to Lightbox" plugin (in operation).
Using keywords for filtering -
thread - This discussion is quite interesting, and I agree with many aspects of both sides (repository-only versus gallery-only). It's relevant to using keywords and may influence how you would add/modify/delete them.
Allowing users to add keywords -
thread - This is not related to batch-adding but may be relevant to the interface used to add keywords, and might be useful as a clean-up after batch-adding (by the admin) or in cases where you batch-add the group keywords, then pick & choose the application of less-used keywords afterwards.
Using editpics.php to mass-apply certain features already -
thread - should batch-adding keywords be done somehow on editpics.php?
Please organize any thoughts you have and post them carefully so this thread does not go crazy.

Think very specifically about how you would ideally apply keywords to your photos. That's why I'm gathering together the threads I've found so far. I might as well incorporate other ideas as I flesh out my own needs.
As this is the features request board, remember that there is
no support here. If you have a specific support question that seems relevant (and is not a feature request), post on the correct board, with a reference to this thread if appropriate. I cannot guarantee everything or anything here although I do have a strong desire to implement this for myself and hope I can share a usable plugin with others.