Do you plan to test intact jackets or just pouches of down extracted from jackets?
You say your aim is to do "real world" testing. By this I assume you mean you want to do your testing under real conditions outdoors. Tests in a humidor or other enclosure, with controlled temperature and humidity, is not "real world" testing.
The problem with real world testing is that the testing conditions are not controllable. It's hard to compare results for different jackets if they were worn by different people during the tests, or by the same person at different times. This kind of test is only valid if you control for the differences in conditions by testing each jacket many times. If you do that, and make your measurements (of garment weight, loft, internal humidity and temperature, etc.) the same way every time, you can minimize the variance and the effects of confounding factors.
If you don't actually want to do "real world" testing, you could construct a temperature and humidity controlled enclosure and get much more reliable results with a smaller number of replicates. Your experimental set-up can be realistic, and do a fair job of simulating real-world conditions. A dummy that perspires at a controllable rate would not be difficult to fabricate from a manniquin and a peristaltic pump, and an old upright freezer could be modified to simulate any climate conditions you choose. It would be a much larger investment in time and money than just getting a couple of people to wear the jackets outside, but you'd have a lot more confidence in your data.