Northstar was OHC variable cylinder... so its been done...and as stated GM cited corvette hood clearance for the push rod direction...
lets face it GM and Chryslers are not shooting stars these days and that has to be a factor in staying with that tech, the vast majority of the automotive world has moved on.
My 5.9 4wd Durango mpg paid for Crawford Ranch... It was cr*p all the way around....
Coolant loss limp home mode is a GM thing (5.3 truck engines have it too), but its only purpose is to theorhetically save the engine when the cooling system fails. It's not for fuel saving.