This would be a good place to start. I think having a [pd META] subpatch will make accurate parsing easier, and allow for other types of meta data to be easily added. That's what we did with the PDDP reference patch.
but in case the externals get organised through function (instead of author/lib folder), the meta patches won't make much sense. how about frank's (standard) idea of using the text files with short description? each author/lib could have his name on that file - jmmmp.txt, hid.txt, etc. Or adding a string, like lib-jmmmp.txt, lib-hid.txt, ...
btw, I never noticed why the meta patches were there. are they already being used in any way, or that's a work in progress?
As for tagging, as long as it is not too hard to add more tags later, I think that we don't have to spend a ton of time trying to come up with the perfect set of tags.
I would add that in general the discussion is slowing up a bit (that might be the normal pace for mail discussions). How about putting a small page with the most relevant sugestions, in order to try to make a decision? Or leave it for the next time enough pd-people meet in the same room (with computers and no beer)?