I'm a bit curious as to why they didn't partner with some established academics in this area (there's plenty of economists who have done excellent research here) rather than start a new program - but it's YC's money, and they can choose how to spend it.
How open do you plan to be about things like datasets/analysis plans and so forth? I think this would be an excellent opportunity to push forward open science http://osinitiative.org/about-osi/. Further, if you allow people who are skeptical of the minimum income to register the objections before you gather the data, you can address them before you proceed - at that point, post-hoc critiques are a lot less credible.