[Exim] Rewriting headers in ACLs?

I'm using exiscan w/Exim 4.30 on Solaris 8 to do virus and spam scanning on incoming mail. Currently, I'm using the "warn" verb to add a subject header in acl_smtp_data:

warn message = Subject: ***SPAM*** $h_subject

which leaves the message with two separate subject headers - not ideal..

Is there a way to have it replace the subject header instead of adding one? I see from the FAQ that I can use a system filter, but this machine is just a gateway and doesn't do any local deliveries, so (I may be wrong here, please correct me if so) I can't use a filter.

TIA,
Steve

--
"No one can earn a million dollars honestly."
- William Jennings Bryan (1860-1925)