This is a really good question. I'm not sure if you can do that..
Can you try configuring the WorkItemHandler via code instead of Using the properties file?
Can you create a Jira Issue to describe the problem, so we can take a look on that?
The drools.workItemHandlers file, if I don't remember wrong in an MVEL file, so that means that we can inject the session to be used as a variable, but as you mention I don't remember an example of that either.