granular authorization for mailadm group?

All-

Has anyone ever modified UW IMAP so that the "mailadm" group feature to
authenticate as one person but be authorized as any other person can be
limited to a select group of users?

I'm looking at doing that now and it doesn't look terribly difficult, but
if someone has already been down this road any tips or patches would be
appreciated.

I need to give out the ability for one or two accounts to authenticate
as themselves but then be authorized as a small/select list of other
people.  The ADMINGROUP/mailadm feature does most of what I need, I just
need to limit its access to the accounts I want it to be able to access.
Creating another group (something like "mailauth" or "mailimp") and
putting the people that can be valid authorization "targets" into that
group and then modifying pw_login seems to be the most straightforward
approach.

Thoughts and suggestions appreciated,

Tim