[exim] Rewrite Rcpt in copied mail

I want to archive every outgoing mail for customer "foobar". This would
be easily achieved by using a manualroute and "unseen" which sends all
mails to the archive server.

The archiving software on the other hand expects all mails to have the
Recipient replaced with foobar@??? instead of the
original recipient.

I learned that headers_rewrite does not work for envelope data for me:
"However, it is not possible to rewrite envelope recipients at transport
time."

So, I need to create a copy of the mail _and_ change the recipient of
this copied mail. To my knowledge this is not possible within a
transport or router configuration. Is there any other way to achieve
this? I know how to manipulate mails within the ACLs but not how to
create a copy at this time.