[exim] exim with virtual users and procmail

Hi

I have a customer which moves from sendmail to exim/courier-imap and thus
from mbox to maildir.
However, I am in the position to help installing this and I would prefer
virtual users, since all users are doing only mail and there shall be no
other access to that system.

BUT incoming mail shall be processed by procmail to remove attachments and
such.
Now, I found in the documentation how to run procmail via pipe transport,
but it seems to require system users.

Has anyone used procmail with virtual users?
Or can someone give me a hint how to do this?

[I do NOT need different procmail configurations, except the maildir folder
to drop the message in.]

Regards,
Steffen