dict_pgsql.c(postfix-2.4-20060903)

638 /*
639 * XXX Postfix does not send multi-byte characters. The following
640 * piece of code is an explicit statement of this fact, and the
641 * database server should not accept multi-byte information after
642 * this point.
643 */
644 if (PQsetClientEncoding(host->db, "LATIN1") != 0) {
645 msg_warn("dict_pgsql: cannot set the encoding to LATIN1, skipping %s",
646 host->hostname);
647 plpgsql_down_host(host);
648 return;
649 }

現状ではソース変更ぐらいしかPostfix側でできることはないと思われ。