Exactly correct! Long ago I wrote the LongBets software and am still involved in the Long Now, so I'm happy to facilitate registering a bet between people on this topic; just email me.
At this point we've been up for 14 years. If we're good enough for Warren Buffet's million-dollar bet [1], I'd say we can handle this bet too.
Any way to use a pseudonym? I'm happy to throw $200 bucks into the pot.
If you can find a counterparty and that's the only way to get the bet done, I'm glad to ask. But the theory of Long Bets is as a registry for accountable predictions. Accountability is tied to identity. So my guess is that they'd say yes for a well-known pseudonym with a body of work, like Lewis Carroll or Voltaire, but would say no for a throwaway name.
I think they are legal, because the winnings doesn't go to the participants, but to a charity of the winners choosing.