While visiting Oxford for MIRI’s November 2013 workshop, I had the pleasure of visiting a meeting of “Skeptics in the Pub” in the delightfully British-sounding town of High Wycombe in Buckinghamshire. (Say that aloud in a British accent and try not to grin; I dare you!)
I presented a mildly drunk intro to applied rationality, followed by a 2-hour Q&A that, naturally, wandered into the subject of why AI will inevitably eat the Earth. I must have been fairly compelling despite the beer, because at one point I noticed the bartenders were leaning uncomfortably over one end of the bar in order to hear me, ignoring thirsty customers at the other end.
Anyhoo, at one point I was talking about the role of formal knowledge in applied rationality, so I explained Solomonoff’s lightsaber and why it made me think the wave function never collapses.
Someone — I can’t recall who; let’s say “Bob” — wisely asked, “But if quantum interpretations all predict the same observations, what does it mean for you to say the wave function never collapses? What do you anticipate?” 
Now, I don’t actually know whether the usual proposals for experimental tests of collapse make sense, so instead I answered:
Well, I think theoretical physics is truth-tracking enough that it eventually converges toward true theories, so one thing I anticipate as a result of favoring a no-collapse view is that a significantly greater fraction of physicists will reject collapse in 20 years, compared to today.
Had Bob and I wanted to bet on whether the wave function collapses or not, that would have been an awfully tricky bet to settle. But if we roughly agree on the truth-trackingness of physics as a field, then we can use the consensus of physicists a decade or two from now as a proxy for physical truth, and bet on that instead.
This won’t work for some fields. For example, philosophy sometimes looks more like a random walk than a truth-tracking inquiry — or, more charitably, it tracks truth on the scale of centuries rather than decades. For example, did you know that one year after the cover of TIME asked “Is God dead?”, a philosopher named Alvin Plantinga launched a renaissance in Christian philosophy, such that theism and Christian particularism were more commonly defended by analytic philosophers in the 1970s than they were in the 1930s? I also have the impression that moral realism was a more popular view in the 1990s than it was in the 1970s, and that physicalism is less common today than it was in the 1960s, but I’m less sure about those.
You can also do this for bets that are hard to settle for a different kind of reason, e.g. an apocalypse bet.  Suppose Bob and I want to bet on whether smarter-than-human AI is technologically feasible. Trouble is, if it’s ever proven that superhuman AI is feasible, that event might overthrow the global economy, making it hard to collect the bet, or at least pointless.
But suppose Bob and I agree that AI scientists, or computer scientists, or technology advisors to first-world governments, or some other set of experts, is likely to converge toward the true answer on the feasibility of superhuman AI as time passes, as humanity learns more, etc. Then we can instead make a bet on whether it will be the case, 20 years from now, that a significantly increased or decreased fraction of those experts will think superhuman AI is feasible.
Often, there won’t be acceptable polls of the experts at both times, for settling the bet. But domain experts typically have a general sense of whether some view has become more or less common in their field over time. So Bob and I could agree to poll a randomly chosen subset of our chosen expert community 20 years from now, asking them how common the view in question is at that time and how common it was 20 years earlier, and settle our bet that way.
Getting the details right for this sort of long-term bet isn’t trivial, but I don't see a fatal flaw. Is there a fatal flaw in the idea that I’ve missed?