[Dovecot] How to add new features to sieve?

[Ed W]

Hi, I like the idea of switching from maildrop to dovecot deliver+sieve, 
however, we use a couple of features that I'm not sure how to migrate:

- We query the userdb and make some user specific params available to 
the filtering script (invented example, but say number of days to not 
repeat for the vacation script)

- We have a fairly advanced filter (perl) which mangles the email in 
very specific ways to support users over low bandwidth connections.  The 
email is delivered twice, once to an "Archive" folder and again to the 
inbox after being mangled.  How to integrate this "mangling" script?


- We have some custom routes for certain types of SMS messages, ie we 
have a free email to SMS option for certain networks.  Could this be 
integrated and caught in "alert" functionality (how is the sms backend 
actually configured anyway?)

I think the answer is to write one or more new sieve functions to 
provide this functionality, but can you give me a quickstart guide on 
how to do this?  Certainly the first requirement is quite an interesting 
and generic plugin "lookup_db" type function which must have wider 
utility.  The second requirement is a bit more specific, kind of like an 
xfilter wrapper which could be setup by the administrator I guess?

Thanks

Ed W