I did consider using a small brick like that, among other hare-brained ideas like installing a small power supply in the rad box
I've finally decided on using a 3.5 inch black Vantec Nexus fanbus. I'll run the power along the coolant hose between the PC and the radiator to help keep things clean-looking. The benefit here is that the fans will tun on with the PC, and I can control the speed of each fan individually. It's more expensive than using diodes, but it offers much more control over fan speed/noise.