If it was up to me, I'd define certain places as being ka'e-able:
that is, "this place is normally filled by X [where in the case of
botpi X would be "what the bottle currently contains"], but even if
there is no current or obvious X, the capability is sufficient for
the semantics of this word; zi'o should only be used if the
capability has been lost".
I don't know if I agree with that. I think a {broda zi'o} isn't necessarily a {broda} that cannot have an x2 but rather a {broda} that in this instance does not have an x2. i.e. there's no reason that {ko'a simlu lo ka barda zi'o} COULDN'T have an observer, only that in this instance it does not. Or... maybe that's the chief difference between noda and zi'o? I'll be honest, my grasp of what zi'o is/does in a technical sense is fairly weak.