I have been able to create my own ForEach jpdl element. Like you said, this wasn't that hard. Just copied the Subprocess jpdl component and binding and changed it a bit so it iterates over the subprocess. Works like a charm.
I would appreciate a bit more info about how to change the GPD so my new jpdl element also shows up in the grafical view. I have looked at the GPD sources and frankly I don't have a clue where to look. If you can point me to where, for example, the Subprocess stuff is and what kind of classes I need to create, I will probably be able to copy/adapt this to my wishes.