Make sure that as7/standalone/configuration/standalone.xml has :
<spec-descriptor-property-replacement>true</spec-descriptor-property-replacement>
This was reported as a bug recently (
https://issues.jboss.org/browse/AS7-5015) that is resolved by setting the spec-descriptor-property-replacement to true. As mentioned in the jira, I was able to pass a property definition when starting AS which was used in the persistence.xml.
./standalone.sh "-Dtest.ds=java:jboss/datasources/ExampleDS"
Hope this helps.
Scott