rejetto,
Would it be possible to add a 'Comment' section to each user account? There is quite a bit of empty space in the existing user account window where this section could go. The admin could then manually add new info about a user into their respective comment section.
For example, for a user to get increased access on my site, they have to register. Part of the registration process is to get their email address which I would then respond to. In an 'Comment' section for the user I could add their email address. That way there is less of a chance I would lose it or get it mixed up with another user's email address. That's just one example of what that section could be used for.
What do you think?