It does work - just like it does for Actions.
All you have to do is add some fields to your AssignmentHandler-class and specify the parameters in the same way as you would for Actions. (At least in v3.0.1 which I am currently using.)
One pitfall might be, that the JBPM-IDE I am currently using doesn't provide a GUI-way to specify the handler and parameters. so I have to manually edit the processdefinition.xml.