Here's a link to the spec/install sheet of that Datel shunt:
http://wbta.us/bigben2k/datel_shunts.pdf
It covers a lot. Yeah it's open, so you don't want to touch it unless you're isolated from ground, especially in your case, because you're looking at a (relatively) high DC voltage, which can be very dangerous. I'd try to cover it up with some sturdy but vented box, plastic or wood (or whatever electrical isolation of your choice).
So you'd measure the dT between air inlet, and water outlet? That seems reasonable. As long as everything is calibrated, and repeatable, you ought to be able to tell which radiator is better, with confidence.
The only issue I see, and maybe you can share your thinking here too, is how (and if) you would address the difference in size of the radiators.
Note Bill's article, here:
http://overclockers.com/articles481/
Bill created a test setup where the pressure drop is the same for all radiators (which of course results in various airflow rates and speed). Of course it makes no reference to actual fans typically used, and as it turns out, some of these pressure drops cannot be achieved with a typical single axial fan (hence why blowers are starting to crop up).
The difference here though is that Bill set the system to maintain a dT of 10 deg C, then simply measured the heat level put into the coolant loop.
The 10 deg C was an arbitrarily set point, as it was (still?) representative of an average water cooling setup. I'm not sure that it would apply in yours though.