Ryan that is a good idea. I had been thinking about a similar type of fix using a pvc ball valve. If you look at the siphon system diagrams that they have on the pond dam piping website they show a 3 or 4 inch tee on top of the riser for the vent pipe with a plug in the top. I had thought about putting a pvc ball valve there and just opening it when we have freezing weather. Usually when it is freezing the water flow really slows down so I believe that may work to prevent accidental siphoning of the pond.

Bob