Dix:
Yes I agree that you have to have a block or two as "standards" that you retest every time you break down/upgrade/change your system. I used a GLaciator2 and a Dynatron BH610 for this purpose when testing heatsinks a while back. It is the only way to have any confidence that your numbers are not being affected by some experimental error. Your proposed setup isn't so different than Bill's except instead of an insulated box he is using a lab chiller for his submersible pump and controls the set temperature so that it always results in an inlet water temperature that is constant. You'd really need more temp probes
but the advantage of your design is that you could cheaply find a digital thermometer with type Ks that has two inputs (Ex Tech?). The problem is the time required (I suspect) to come to an equilibrium, and the fact that different blocks will have different restriction amounts, and so the pump will generate different amounts of heat (how to correct?). Flow rate data still needed, and also much more tedious for this since your pump will become more and more important in heat generation.
The thing that Bill and I have been kicking back and forth here is that most of the die simulators (Bill's excluded) seem to still have substantial secondary losses as well (or else their calculated W isn't much better than radiate's). So you are back to only internally valid numbers; no comparison between testers is good since their error in W created vs. W applied is all going to be different anyway.
No comparison between different testers is ever likely to be good; testers might as well just shoot for internally good numbers and then explain their methods. The TIM between die and block is a huge variable, as is (for most) mounting pressure. I can probably get a consistent paste application over time, but I wager that it will never be the same thickness settled upon by other testers.