[exim] pipe transport... or similar

Hi all...

This is weird... can I somehow use any existing transport to execute
dspam command to retrain a msg? The catch is... without really piping
the msg through dspam

Basically, the msg was already filtered, and dspam added a header with
the signature of the msg. Now I just want to execute the command:

dspam --user $local_part@$domain --class=spam
--signature=$header_X-DSPAM-Signature

But I dont want to pipe a 50MB msg through it for example.

Thanks for any hint.