if toolmaker has been long enough on the net, he'll know that the corsair H series modding circle have been able to maintain the closed loop look and also not add any new hardware to the equation.
when the pump housing has been disassembled, there are a few points along the pump chamber that allows one to bleed the loop with a syringe and also add liquid/coolant in through the same hole.
in fact, if patient, you can pull this off with only a length of tubing and syringe. No rad needed and while the pump is running - remove air and add liquid to the loop. Soon enough you have a closed loop with the pump.
all this is doable, but not worth it IMP as you can add a valve - hide it out of sight
http://www.overclock.net/t/629298/mod-corsair-h50-with-240mm-radiator, principle same, just pump changes from antec-corsair-asetek
http://www.evga.com/forums/tm.aspx?m=85534&mpage=1