PAM - rewriting usernames

[Ben Chabot]

Hello!

I'm interested in a writing a module (or modifying an existing module),
that will take a username like user at domain.com and using a database,
rewrite it to user_##### and login with it. Or, if the username entered
is not an email address, proceed with a normal login.

So far, I think the FTP module comes closest to what I'm looking for, as
it detects a different type of username, does something with it, then
continues login, am I correct?

At any rate, I'm wondering if you kind folks might have a better example
for me, or know of a really good book on PAM.  Also, criticisms of this
idea are welcome.  Is there something I'm missing that makes this
impossible, or a foolish thing to do?  My hope is that this has been
done already, in some form or another, and I just haven't seen it yet!
That would be great :-D

Any help would be greatly appreciated!!
Thank you,
Ben