In most cases, DDL is applied manually after careful thought, so people seldom dump at the same time they upgrade the database. This is especially true for pg_dump since it captures the logical definition of tables. So most people will be happy with the default locking, but we could make the lock level optional.
Currently we use AccessShareLock. Locking out all DDL, even with this patch applied would only require ShareUpdateExclusiveLock.
Looking at the code, it will take about an hour to add an option to pg_dump that specifies the lock level used when dumping. I would be happy to include that as part of this patch.
I think the use case for specifying multiple locks is pretty slim given that a ShareUpdateExclusiveLock is good enough mostly for everybody.
If its not the case, the user should be more careful about when he is scheduling backups to so that they dont conflict with DDL changes.
I am not too comfortable with exposing the locking type to the user. That may be just me though.