Limiting WIP in the upstream kanban is actually the wrong approach. Teams need a minimal set of options to choose from at all times in order to prevent starvation from high value work (any work cycle that is lost by the team on not working on high value work is lost forever).

The upstream kanban needs to implement a triage process with different opportunities for selection. Just like in a real world triage process the purpose is to ensure that the amount of effort that is put in an item is in line with the degree of urgency and chance of survival. At each step we do need sufficient options to select from. Think of a sales pipeline. At each step in the pipeline you need sufficient options to compensate for those that are selected away or that get stuck.

That being said, sometimes it does make sense to implement an end-to-end CONWIP to ensure a good overall flow. I will publish on that later.