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?