Ok, wow...
I think I just accidentally stumbled upon a (partly) solution, when reading this thread.
As all assigned users to tasks under a project can also be queried using the projectUsers collection, you can use something like this:
valueexpression=CONCAT({user}.{role}, " - ",{user}.{name})
listdelimiter=<div>
listmethod=nested(projectUsers).lists
valueformat=HTML
displayname=Assignees
description=Assignees
textmode=true
type=iterate
I am quite sure, that the limitation, you will need to live with is, that you will only get the primary role of the users.
Regards
Lars