Hi Jannis
You are right. this would also be an opotion (I will probably need to create separate UI self-service task for each SAP platform anyway).
I analyzed pros and cons and it was a "close call". I have, however, decided to go for using the UserID field and were able to succesfully determine which attribute change has triggered the modify task based on the Attribute ID provided there. Switch task works well and this setup appears to be very flexible and future proof.
I am not changing standard provisioning framework 
those action tasks executed after the Z_INACTIVE.... attribute change are custom action tasks and they are assigned directly to attribute as Add and Remove event tasks.
What I just might need to do is to customize "modify ABAP user" onnector so that it did not update VALID TO dates for users that are inactive on relevant systems, but since we have all connectors customized anyway, I'm not that concerned about it