You could put your flow inside of a jacket flow, as a subflow. Then when your flow hits the end shape and unwinds into the jacket, you could have a utility there that calls your activity. That's probably the most straightforward way to do it.
I have used a subflow in a flow, then used a utility(to call a activity) after the subflow shape, but that sub flow doesn't commit itself. When the main flow completes ,then subflow changes will be committed(one commit), therefore the called activity cannot be done the work as it is expected (some subflow changes are required to have in that called activity).
Please let me know your thoughts as this is a high priority work in my project.We can have screen sharing session if can, let me know.