I've also tried to find a way to externalize the maxSession property out of ear.
But had no success with this solution under JBoss 4.2.3.GA. DefaultActivationSpecs were not taken into account.
Extensive googling gave a hint to alternative solution with using system properties in ejb-jar.xml. It works with all versions before JBoss AS 7.
So now I can just update property in deploy/properties-service.xml then touch ear to redeploy and new setting gets applied.
No full JBoss restart is needed.