use of websphere variables.

We are trying to run EntireX under Websphere on the z/OS mainframe.
In WEB.XML the definition of the XML-INIT.XML can be set.
We want this to be done by a websphere variable like :
This seems not to work. The reason why we want to do this is that we then can use the same EAR file for all environments.
Now i have to make one EAR file for each environment.
Is there a way to use the variables in WEB.XML or is there another way ?