dovecot backport for bullseye

I'd like to upload a backport of Dovecot 2.3.21 to bullseye-backports.
There are a couple of complicating factors on which I'd like some input.

1. There are two additional source packages that build plugins targeting
dovecot (dovecot-fts-xapian and dovecot-antispam).  These need to be
rebuilt against the new dovecot ABI.  Normally this is done via binNMU
in unstable.  How is this handled in bpo?

2. To complicate things, there is already a version of
dovecot-fts-xapian in bullseye-backports.  This is built against the
version of dovecot in bullseye.  If we rebuild it against the new
version I'd like to upload, it will no longer function with the bullseye
version, and would effectively force people to upgrade to the new
dovecot packages in order to keep using it.  That may be undesirable.
What's the right approach here?

Thanks
noah