I bought one of those 2.5/3.5 to USB adapters so I could salvage some pictures from the HD in my wife's old computer. Since then, I've used the AC adapter (which has a molex plug on the end) to prime my w/c loop with the PC itself off. Then, in the middle of a fit of insanity (or was it insomnia?)...it hit me..
I could use a DPDT switch to select either primary PSU power or the small adapter PSU to power the pump. I figure with the DPDT switch, it's either primary or priming PSU with no risk of over powering or shorting anything.
Throw in some strategically placed drain and fill ports, and you could flush your w/c loop, refill, and prime without ever opening your case.