After digging into this problem I concur with Pinaki's assessment that the lock modes / lock configuration has numerous issues. I'm posting a patch that will only address the backward compatibility issue.
The root issue at hand is that in JPA 1.0 a named query had a default lock mode of READ (by OpenJPA's definition, not the spec), but per the 2.0 spec the default lock mode is NONE.
Ideally when we are parsing annotations in AnnotationPersistenceMetaDataParser we could differentiate between a named query which has specified the lockMode vs a default lockMode. Since that doesn't seem to be possible, I'm proposing that if we detect that we're using a pessimistic lock manager and the lockMode is none, we will promote that lock to a READ lock.
This approach will restore backward compatibility, but will exclude one use case. When using a pessimistic lock manager on 2.0 you will be unable to configure a NamedQuery with the lockMode of NONE.