I have come across the following construction in an educational exercise book written by a team of non-native teachers.
I say she will be OK, won't she?
Shouldn't the tag question be "don't I"? To my knowledge, in this type of structure, the tag corresponds to the main clause.
There are different ways that "I say" could be used at the beginning of a passage of speech. It has an old-fashioned, predominantly British use as a way to express surprise (when it would be written as an exclamation) and also to attract attention. The punctuation of your sentence does not allow for either of these.
The other use of "I say" is synonymous with "I think", and used as a preface to stating an opinion, or something one believes to be a fact. I would have to guess this is what was intended.
It doesn't make logical sense to use the expression "I say" to indicate certainty of belief or opinion, and then to add a tag question seeking confirmation. So, your example is not particularly idiomatic.
However, ignoring these opening words - the general rule of tag questions is that they must have polarity with the statement they follow, typically using a form of the same verb, for example: "You do like it, don't you?". So, your tag question of "won't she?" is correct for the statement "she will".