you would have to introduce a glycol solution / AKA antifreeze
take the rads out of the loop
I woould also keep the flow as slow as possiple to maximixe heat transfere at the block, but yes it would work under those circumstances ........
you will probably create condinsation so insulating the cards would not be a bad idea
but it depends on what type of heat transfer you get across the block and flow will determin
I would have to think with a 20 run time you could drop the loop temp considerably
it may have to catch up between benches as 4 of those will put off some heeat