I'm routinely vacuumlo'ing to reap orphan OIDs. Is it necessary to manually vacuum pg_largobject or is it handled by autovacuum?
It is handled by autovacuum. What we do is, we schedule a manual VACUUM ANALYZE nightly job on bigger tables to avoid burden on the autovacuum during the business time.
In a system where large objects are constantly added (and *some* rarely deleted, so it grows every day), would I gain space (freed to the OS) by VACUUM FULL it?
The amount of reclaimed space will depend on the volume of deletions happening. If the DELETES are rare and are not deleting much, then frequent VACUUM FULL is not ideal.