All the clever ways of getting betting markets to take xrisks into account suffer from one big flaw: the rational xrisk bettor only makes money if the xrisk actually happens.
Now, the problem isn’t because “when everyone is dead, no-one can collect bets”. Robin Hanson has suggested some interesting ideas involving tickets for refuges (shelters from the disaster), and many xrisks will be either survivable (they are called xrisks, after all) or will take some time to reach extinction (such as a nuclear winter leading to a cascade of failures). Even if markets are likely to collapse after the event, they are not certain to collapse, and in theory we can also price in efforts to increase the resilience of markets and see how changes in that resilience changes the prices of refuge tickets.
The main problem, however, is just how irrational people are about xrisks, and how little discipline the market can bring to them. Anyone who strongly over-estimates the probability of an xrisk can expect to gradually lose all their money if they act on that belief. But someone who under-estimates xrisk probability will not suffer until an xrisk actually happens. And even then, they will only suffer in a few specific cases (refuge prices are actually honoured and those without them suffer worse fates). This is, in a way, the ultimate Talebian about black swan: huge market crashes are far more common and understandable than xrisks.
Since that’s the case, it might be better to set up a market in near misses (an idea I’ve heard before, but can’t source right now). A large meteor that shoots between the Earth and the Moon; conventional wars involving nuclear powers; rates of nuclear or biotech accidents. All these are survivable, and repeated, so the market should be much better at converging, with the overoptimistic repeatedly chastised as well as the overpessimistic.