[exim] conditional local_scan()?

Hello,

I would like to run an antivirus protection on an incoming mail if the
appropriate user of the target-mailbox has asked us to do this. I want
to use local_scan() for so that the message acceptance can be denied
directly after the DATA-command and the mailbody so that the message is
not accepted if a virus is found. Is this possible? exim-expressions are
very powerful and I would like to use them instead of putting very
simple expressions within my local_scan()-function (where mysql or
ldap-calls would bloat the code).

Regards
Marten