Eg. you have an objective prior in mind for the Ellsberg urn, presumably uniform over the 61 configurations, perhaps based on max entropy.

Well, that would correspond to a complete absence of knowledge that would favour any configuration over any other, but I do endorse this basic framework for prior selection.

So we could normally start the state variable at the "natural value" (virtual interval = 0 : and, yes, as it happens, this is also justified by symmetry in this case.)

Doesn't an interval of 0 just recover Bayesian inference?