Thanks Tihomir.
Unfortunately that doesn't work, I get a NullPointerException at org.jbpm.bpmn2.xml.TaskHandler.readDataInputAssociation(TaskHandler.java:120).
My reading of the code suggests <transformation> is not implemented in jBPM 5.2.0_Final. To me, it seems readDataInputAssociation() only handles <assignment>. And indeed, <transformation> is not listed as supported in chapter '6.3 Supported elements' of the documentation.
The code looks like it's setup to always treat the <to> and <from> as an XPath expression, so I guess I'll have to go through the code to find out what it can do and how to do it. Unless someone can point me to some documentation..?
Thank you for your suggested alternatives, but to be honest, I don't really like them. My goal is to make the process to be as 'business process' as possible, so a task to prepare the parameters for the next task is not in-line with that goal. I'm only using domain-specific tasks, but I don't think it's their job to prepare their own input and output parameters from whatever format the caller uses. I think there should be a way to map tasks' inputs and outputs in the process itself, and that's what I'm trying to do with these dataInputAssociation/dataOutputAssociations.
I will update this discussion thread with any progress I may make. In the meantime, any other suggestions would be most welcome.
Thanks again.