I would say it’s looking good. The dovecot package in 4.3 says “Candidate version 1:2.2.27-3+deb9u2”. In combination with the corresponding documentation http://documentation.open-xchange.com/7.8.4/middleware/components/search/crossfolder_fts_in_mail.html this should be possible.
Now you just have to wait for OX to be released for 4.3 (-> UCS 4.3 Upgrade für OX App Suite?)