Inconsistent use of slashed zero?

I'm not sure I agree. The tradeoff only exists in the abstract. If in a single display a user sees some round shapes, and some round shapes with a slash, then they could, on the basis of no other information, conclude that all of the former are the letter "O", and all of latter are the numeral zero. It requires more scrutiny and reasoning about the implications of various field types and values to carry out the logic of readability you describe. Good design should not so burden the user. The obvious interpretation should be the interpretation. Note that the set of allowable values is not even clear from a field name: sometimes a license number is inaccurately named and can contain letters.