Hi Jaber,
If you want to implement your own function, please refer org.jbpm.pvm.internal.el.JstlFunction. And I think Micheal's codes is very helpful.
BWT, I think custom el function feature is very cool. But there were lots of works on EL which Tom have not completed yet, I am afraid we may not finish them on jbpm 4.4. Maybe on the next release we could make a good plan to finish these unresovled issues.
Thank you very much.