Although the topic was not quite the same, I recently posted an extremely small proof of concept module that demonstrates this. As Ratty said, everything hinges on the use of the Map window-level setting where you can specify a keystroke to be applied to all pieces ending movement on the map. That keystroke fires a Trigger on your piece, and the Trigger uses a property match expression to basically query if the piece is now in a certain zone and wasn’t previously (to avoid spurious activations of the Trigger from someone reorganizing pieces or moving things around in the zone).
Thread: Can you count pieces in a zone?