Crazy idea here, probably a million reasons why not to but I'll post it anyway.
Why not have a cyclic secret that is placed in the main list's signature (the one that appears on every main list posting) Surely the only people who have a legitimate interest to modify the wiki are already list members.
If it's a simple matter of checking the bottom of one of the recent main postings I think it's something we could live with that provides less of a barrier than yet another registration form.
A bot isn't going to be clever enough to strip the secret from the list archives, perhaps a human could be but we could conceivably strip it from the archive if that was a concern.
Another option, if we are going to go with usernames/passwords. can we set it up so that the Library uses the same registration database or vice versa...seems silly to duplicate registrations amongst the same group, I am happy to provide whatever is needed from the Library end.
Wayne