[exim] Lookup in rewrite rule

I am testing a new server, which has the same user and auth data as the
production server. I need a way to make sure that mail for my beta
testers stays on the beta server, while everything else goes over to the
production server. Since I am currently the only user, this rewrite
rule has done the trick:

*@cse.psu.edu "${if !match {$1}{^soccio} {$1@???}fail}"

Where psuvax1.cse.psu.edu is the production server. I am at the point
where I need to start adding my beta users, so I would like this to be a
file lookup, but I am having problems figuring out how to rewrite upon a
failed lookup. I'd much rather keep a small list of users that don't
need to be rewritten, than have to keep a list of all users on the
production server minus a handful of beta testers. I am implementing
plus addressing, so I need be able to match even if $1 contains "+foo".

Some things that I have tried, but do not work, assuming
/etc/exim/betausers contains ^soccio:

*@cse.psu.edu "${lookup{$1}wildlsearch{/etc/exim/betausers} {}
{$1@???}}
*@cse.psu.edu "${!lookup{$1}wildlsearch{/etc/exim/betausers}
{$1@???}fail} < I've moved that "!" just about
everywhere I can try.
*@cse.psu.edu "${lookup{$1}wildlsearch{/etc/exim/betausers}
fail{$1@???}}

Can anyone help with the syntax to fail upon an *lsearch lookup match
for rewrites?

Thanks

Matt