I looked into the source code for some delegates regarding world partition streaming and after some searching I found this blueprint callable function in the subsystem
I’m guessing it tests based on set coordinates and radius. With a small enough radius it probably targets the cell at that location.
Shame there is not delegate that you could hook into that would broadcast it’s change. Perhaps it would be an idea for a pull to the source?
From a c++ side you could override void UWorldPartitionSubsystem::UpdateStreamingState() as it’s virtual
This could be used to call on updated state. I would have to read up on extending sub systems with custom properties.