Has the concept of UBI been around long enough to fulfill your requirements? A 20-year study across a large population would of course be superior, but shorter-length studies with less people are necessary to prove/disprove whether those large scale studies should be funded. Not to mention the ethical implications of forcing someone into a large scale study like that before any results have been shown at all.
I think it's fine to be skeptical of anyone considering UBI to be "case closed", but small studies being done before large studies is standard practice. You can't give that kind of grand scale funding to every hypothesis that pops into someone's head, so it's a reasonable way of determining what shows promise and should be looked into more.
I suppose you could do it a la "The Dispossesed" where you spend one day/week doing community service