> I'm not even sure how a frequentist would construct a model to estimate an outcome with no observations.
The same way a Bayesian would, since it's a question about probabilities of hypothetical experiments, not about statistics. Or you go through decision theory instead of mucking about with half baked ideologies.
> being Bayesian is modelled on how humans process and contextualise decisions.
This is false.
Unless you have reason to believe I am trying to deceive you, it will be able 50% because you have a lot of knowledge from other contexts that tells you this is true.
The arrow is probably the other way round than you state -the brain probably isn't Bayesian; being Bayesian is modelled on how humans process and contextualise decisions.
I'm not even sure how a frequentist would construct a model to estimate an outcome with no observations.