If all you need to do is lock a schema, you can just call LockDatabaseObject(NamespaceRelationId, namespace_oid, 0, AccessShareLock); there's no need to fake up an objectaddress just to take a lock. But I think that's not really all you need to do, because somebody could drop the namespace between the time that you decide what OID to lock and the time you acquire the lock. So I think you need something like what we did in RangeVarGetRelid(). See attached patch.
Thanks Robert. But currently there are very few callers of RangeVarGetAndCheckCreationNamespace() function. For the sake of completeness we will have to introduce a call to this function while creating all other objects too.