Yes, that is certainly a challenge. I assume you know that there are several systems on the market for that, but they are most likely a little bit more expensive
I checked the movie and unfortunately that yellow label can be put anywhere. Some suggestions:
-first use the Detection Zones in the menu for Motion Detection. Block everything that is not relevant.
-test the algorithm for blob detection.
-adjust the Threshold and see if you can start the motion detection when the object pass into the detection zone.
-experiment with the Rule manager if OnMotionStart a recording will start or a Timelaps will start to take a number of pictures until motion stops..
A really interesting application, but challenging.
Good luck and I am looking forward to a report how it goes.