The harder challenge seems to be the working on the shared folder and
leave the inbox untouched!
I don't see why that would be a problem, in general.
The real problem you have here is that Sieve won't do it as currently
defined. That's because it's only invoked during mail *delivery*, not
during an IMAP operation, such as APPEND or COPY. It seems like you
need it during the IMAP operations.
Watch this spec: draft-ietf-lemonade-imap-sieve
Assuming I ever get the time to get it finished (it needs a script
management piece, using mailbox metadata, which I'm behind on getting
defined. Once it's ready, lobby the Cyrus folks to implement it, and
then you can write a Sieve script to do what you want.
--
Barry Leiba