Interesting concept. In theory it should work. Just a couple suggestions...

1. What if you lose power or a pump? Anyone who uses this method should make sure that the entire auto-top reservoir does not overfill the system, as all of the water will drain out without a pump or in case of a power failure.

2. I would be curious to know what the circulation rates are through the auto-top res. Maybe someone is ambitious enough to crunch the flow and static pressure/circular pressured weir equations.