Ah, now I actually see/understand your problem -- at least I think so. :-)
My previous suggestion is wrong.
What you need is the following, in theory at the moment.
* deployment myapp.ear must depend on SystemPropertiesService, but in PARSE stage
** this way ear' -ds.xml will be parsed with required system properties
We already have proper mechanism in place for this,
see jboss-dependency.xml (http://community.jboss.org/wiki/JBoss5custommetadatafiles),
but it will need minor adjustments.
See dependency-deployers-jboss-beans.xml in deployers/,
all three jboss-dependency.xml related deployers need a change of stage.
<property name="stage">DeploymentStages.PRE_PARSE</property> <--- pseudo code
Then adding proper jboss-dependency.xml into ear' META-INF should do the trick.