You might as well ask the members of Strigli as they land on Earth what their probability is that the Red Sox will win at a spelling bee next year - does it look obvious that they shouldn't say 50% in that case?
Unless there's some reason that they'd suspect it's more likely for us to ask them a trick question whose answer is "No" than one whose question is "Yes" (although it is probably easier to create trick questions whose answer is "No", and the Striglian could take that into account), 50% isn't a bad probability to assign if asked a completely foreign Yes-No question.
Basically, I think that this and the other problems of this nature discussed on LW are instances of the same phenomenon: when the space of possibilities (for alien culture, Omega's decision algorithm, etc) grows so large and so convoluted as to be utterly intractable for us, our posterior probabilities should be basically our ignorance priors all over again.
It seems to me that even if you know that there is a Doldun game, played by exactly two teams, of which one is Strigli, which game exactly one team will entirely win, 50% is as high as you should go. If you don't have that much precise information, then 50% is an extremely generous upper bound for how likely you should consider a Strigli win. The space of all meaningful false propositions is hugely larger than the space of all meaningful true propositions. For every proposition that is true, you can also contradict it directly, and then present a long l...
We've had these for a year, I'm sure we all know what to do by now.
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