Why flow sensors? Why not measure temperature on whichever device will get hot the fastest if pumps or radiator fan fails (might be CPU, might be GPU). You'd still have plenty of time to shutdown gracefully, wouldn't impede flow (the more "oomph" you need to use to circulate your water, the louder things are going to be) and would involve no moving parts.
on my wc setup, i just set it so that if the cpu temp goes over 70C it shuts down. i tested it with my air setup, unpluging fan and running fah, and it kills power fast enough. havnt tested it wiv the wc setup , but it does give me peace of mind.
edit: soz, didnt relize how old this thread was.