Well, rather than a full blown sendmail/exim etc install would this do?

http://search.cpan.org/~macgyver/SMTP-Server-1.1/Server/Relay.pm

and then s/To:.../To:[email protected]/
(which looks like it might be trrvial)

Set it to run on a new port (2525?) and use 'smtphost yyy.yyy.yyy.yyy/2525' in fetchmail