The java,hql,sql activities should be able to receive execution context variables

As I read from the documentation it's possible to declare parameters for the java activity.
I would like to pass context variables to the activity (like a document id, set at startup).

Instead of passing "static" values like:
<java .....
<arg><string value="Hi, how are you?"/></arg>

I want to do something like this:
<java .....
<arg><var name="documentId"/></arg>
or
<java .....
<arg>#

</arg>

More infos:
http://www.jboss.org/index.html?module=bb&op=viewtopic&p=4230979#4230979
http://www.jboss.org/index.html?module=bb&op=viewtopic&p=4233028#4233028