[exim] Hard-link Message Copies with Exim?

Using Exim 4.60, we're investigating ways to create hard-link copies
of messages delivered by the "appendfile" Maildir transport. For
example, immediately before moving a message from "tmp" to "new", a
hard-link in a "bak" directory might be made.

This could allow us to backup mail that might otherwise be destroyed
(or accidently deleted by a user) before a nightly backup can be
done, without the storage expense of double delivery.

Has anyone accomplished this before? We wonder if one of these
ideas might do:

Compiling exim with Perl support and using "link". However, Exim
doesn't seem to populate a "filename" (nametag) variable for use with
"Exim::expand_string", and we have no use for perl otherwise.

Use a transport_filter.

Use "unseen" in the local router, then a second router to create a
link somehow. We're worried about the race condition this would create.

Create an Exim patch to add a 'link' before "Urename" in appendfile.c.

Any suggestions appreciated,

Rich Sandberg
richs@???
Systems & Network Administrator
Whidbey Telecom Network Operations