Wow - you've got nerves of steel. There is no way I'd set up a tank with closed loop bulkheads in the base of the tank. If anything goes wrong, your tank will be empty in no time.
Check valves would keep the water from draining in the returns (for a while, until something gets in there and obstructs them - like grains of sand), but you can't put a checkvalve on the drain side.
I do hope it works out for you, but when it comes to this option, I'm just a big scaredy-cat. I won't even use the closed loop bulkheads on the back of my tank because I feel they are too low and would drain my tank about 60% if a pump leaked or the plumbing failed.
Is that you standing in the tank?