Thanks to both for your answers!
The users maintain and change their substitution rules on their own, so I can't use predefined groups here. With seperate notification activity the user receives two emails if no substitute is configured and I also can't send a direct link to the task with the new notification.
I suppose, this is just another missing / nice-to-have feature in SAP BPM. I hope SAP provides more UWL and substitution features in the future...
Thanks and regards,
David