[exim] Advanced rewriting tricks using a database

I have an interesting project and just wondering if Exim can do the
efficiently.

A domain that we'll call domain.com wants to let users communicate
anonymously between members. Each member has an online name and a real
email address as follows

tom@???: batman
bill@???: hulk

So when tom@??? emails hulk@??? the addresses are rewritten
so that the sender becomes batman@??? and is sent to
bill@???.

When bill@??? replies to batman@??? the sender is
rewritten to come from hulk@??? and is delivered to tom@???.

I think the translation file is big but I think a berkeley db file will
do it. Can rewriting handle the translation or would several ACLs do it
better? Just wondering if anyone has done this before?

Thanks in advance.