Yeah, essentially using a filter to work on existing emails would help but I gues what I'm talking about is probably one level of user convenience above that.
Imagine a UI that will let you specify easily email addr to folder mappings:
user1@domain1.com -> User One
user2@domain2.com -> User Two
etc
Now if I just select everything in my Inbox and hit a shortcut or button (auto-file), it would search my Inbox for email from senders with mapped emails and move those emails to the appropriate folders.
The problem right now is that I have to create a filter for each person. The other way I've seen this done is to automatically "auto-file" based on the sender's name in the From header "John Q Daly" goes into folder "John.Q.Daly" under people.
So, I'm pretty sure I can't do this through a zimlet.
Also, I'm not sure how useful it would be to other people. I tend to have problems where I let emails accumulate in my Inbox and then after a few hundred are present I want to auto-file most/all of them.