> tu'o is used just in those places where a number is > grammatically required, but none is wanted semantically.
Well, that's *one* use of tu'o, but need not be the only use of it. So I think it can serve as zo'e as well as zi'o.
Since there are two competing meanings, and they are incompatible in some contexts, we should split {tu'o} into two cmavo, tu'o1 for the zi'o-meaning and tu'o2 for the zo'e-meeaning. I propose {tu'o} should be tu'o1, because it is the one that has seen more use, (including the example in CLL) and tu'o2 can be assigned to {tu'oi}.