[exim] Per-User Filter

I just learned about Exim's filtering language.

I would like to implement a 'per-user' filter, on that runs from a redirect
router if some conditions are met.

Problem is none of the users are actual local, there is no mailbox on the
server. The server is just a forwarder to other servers.

When I try and allow_filter and include file = /some.filter it will fail
because it can't locate the local user.

The router is a redirect router, I have the email I want to redirect to and
would like to use a filter to do a few things like:

Alter the subject
Add some records in a log
Inject a header in to the message

Just without an actual local user I can't figure out how to call the filter
file.

Anyone have a tip on where I should be looking?

Robert G.