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.

Hm. For actual aliens I don't think even that's justified, without either knowing more about their psychology, or having some sort of equally problematic prior regarding the psychology of aliens.