Yeah that would be ok but I was really asking the question in Joe's "heatsink context", as with your probe placement in the w/b it still wouldn't help save the CPU if the waterblock fell off.
This to me is possible but very unlikely and to be honest if a heatsink or waterblock does fall off then a dead CPU is probably deserved .
I think what you and I are trying to prevent is Pump failure death. In my system if waterflow stops, the block and coolant in it heats up fairly slowly and SHDN works fine. Lack of coolant in the block could be borderline though so I will look into a separate shutdown controller it is a good idea. I will however have reservoir low level warning and pump flow warning via the VGA smart doctor when I get around to it.