I have same problem. I am writing an extension function to get a variable(string array type) and add a string value to the end of the array.
I get a List from call function's first argument but it is always empty. When I add an element to list and return, no set occurs in list.
<from expression="add2Set(arrVar, 'testStr')"/>
Another problem is I can not use query="/part/items" type queries,  are not accepted and predicate can not be used exception is given by jbpm.
If anyone find a way to set a value to array, please mail me also.