I agree with Randy that ACAP is the best place to store sieve
scripts. We could define a small subset that an ACAP server must
support to handle Sieve scripts which should be straightforward to
implement and place no large resource requirements on servers.
Locating the ACAP server would be interesting. I would suggest
connecting to the ACAP port on the same host as the IMAP server, but
paying attention to referral. (This complicates the client
implementation somewhat, but is probably worth it.)
I could probably produce a draft of such functionality (and perhaps a
prototype server) if people desire.
Larry