Sam,
Not a 100% positive what you are doing. Are you going to put an overflow box (with teeth) inside of your tank. Then create a U-tube that leads straight down to the sump? If that is what you are thinking - then it won't work. The siphon will be broken when the return pump is turned off.
As far as clogged system and creating a mess, just create a system that is fail safe both when the pump fails or the overflow gets clogged. In my current system, if for some reason the overflows got clogged, I doubt it will over fill the tank. Like wise, if my return pump fails, my sump can easily handle the water plus about another 5 gallon.
Minh