[Dovecot] LDA Question

[Jeff]

I need some advice about replacing maildrop with the Dovecot LDA. I need 
to replicate a setup that uses postfix+maildrop+spamassassin. We'll be 
using Dovecot 1rc13.

In the current setup, maildrop reads a file called .mailfilter in each 
user's folder, then call spamassassin according to the user's 
preferences. The file is simple, and looks like this --

if ( $SIZE > 20971520 )
{
EXITCODE=77
echo "Your email was rejected because it exceeded limit of 20 MB"
exit
}
if ( $SIZE < 26144 )
{
   exception {
      xfilter "/usr/local/bin/spamc -u user at domain.com"
   }
}
exception {
       to "$DEFAULT/Maildir/"
}

What is the most straightforward way to have 'Deliver' do the same 
thing, without changing the way that SA gets called? Apologies in 
advance if this is a boneheaded question. I've been sorting through 
Sieve documentation and HOWTOs and I see everything but what I need.