It is not (ever) necessary to manually specify the activation-spec-class for a message driven bean in openejb-jar.xml.
Once the resource adapter has been identified, the ActivationSpec must go with a particular messagelistener-type. The messaging-type is always configured for the MDB (if omitted from ejb-jar.xml, it specifically defaults to javax.jms.MessageListener), and the messagelistener-type must be unique in the inbound-resourceadapter (due to messagelistener-type-uniqueness). Given the resource adapter and messaging-type, the ActivationSpec is determined as well.
The resource adapter must still be specified in openejb-jar.xml unless the MDB uses a message-destination-link in ejb-jar.xml (in which case the adminobject linked to identifies the resource adapter). Thus, the resource-adapter-name should be optional in openejb-jar.xml, and a runtime check should be added that throws a deployment error if the resource adapter cannot be identified (no resource-adapter-name in openejb-jar.xml and no message-destination-link in ejb-jar.xml)