I am not familiar with IDM, but the IDM integration work is not a main part of jBPM4. So I think you could implement your own descriptor and plugin into jBPM4. You could reference the existing component.
Would this be of any relevance to others/future releases?
In other words: If I success, should I contribute an implementation based on PicketLink back to the community?
If yes I might need some guidelines...
Every helping hand is welcome. I think it is definitely worth it to try and integrate with Picketlink and contribute it back. I think Alejandro is planning on coordinating future 4.x releases so we'll need his input on this for jBPM 4. In the meantime the discussions regarding jBPM 5 have started on our development mailing list (firstname.lastname@example.org). It would be good if you participate in that discussion to make sure this effort will also enhance the next generation of jBPM.