To whom it my concern: i've upgraded my local postgres installation of OpenACS 5.9.0 from PostgreSQL 9.4 to to PostgreSQL 9.5.0, which was released a few days ago, based on the default configuration from mac ports. Everything went smooth. Just dump/restore of OpenACS was sufficient:
# Dump all databases from pg 9.4
PGBIN=/opt/local/lib/postgresql94/bin
sudo -u postgres $PGBIN/pg_dumpall > db.out
Then stop pg 9.4, start pg 9.5, then reload the dump:
# Restore all databases and roles from dump into pg 9.5
PGBIN=/opt/local/lib/postgresql95/bin
sudo -u postgres $PGBIN//psql -f db.out postgres
voilà, start OpenACS, and everything works fine.