Add a SystemPropertiesService definition with URL http://community.jboss.org/wiki/PropertiesService.
Define a properties file in a url
Define a property in the file "conn.string=yourdbconstring"
Add a ${conn.string} in one of your *-ds.xml for one of the properties (like connection string)

This works in JB 4.3 and earlier
This does not work in EAP 5

IT appears that even if this is done in conf/jboss-service.xml the properties do not get bound into the DS.