I’ve found yet another German scholar who prefers the Genitivus qualitatis of ‘evpistolh. Cristou/’ in 2 Cor 3:3. Namely, Franz Zeilinger, in his massive Krieg und Friede in Korinth. It must be in the Sauerkraut. He reasons that God, not Christ, is implied as the author by the nature of the ground texts referred to in Jeremiah and Ezekiel (cf. Vol. 2, p. 72).
Incidentally, as you know, I’ve been talking about the subjective and objective genitive the whole time. But evpistolh. is hardly a verbal noun is it? In other words, we are better of speaking of a genitive of authorship, and a genitive of quality. In fact, I suspect that this genitive in 2 Cor 3:3 conveys not only Christ as content or only as author, but both. In other words, we have a kind of non-verbal plenary genitive (cf. Wallace’s Grammar on the plenary). You know I’m right.