Good question.  No, we are running DsXmlDeploymentInstallProcessor during the Phase.FIRST_MODULE_USE (same as when the first PU phase starts).  The PersistenceUnit service has a dependency on the datasource service, so, ds.xml deployments should work with the two phase PU bootstrap (DeployedXmlJpaDataSourceTestCase is passing without any special hacks, so I think we are good. :) 

Its also easy to disable the two phase PU bootstrap by setting "wildfly.jpa.twophasebootstrap" to false in the persistence.xml.