I'm afraid I have nothing mindblowing to add, only the following which will hardly come as a surprise :
- John 20:28 is clearly an address to Jesus (apekrithè Thômas kai eipen autô, literally "Thomas replied and said to him").
- John 20:28 holds a climactic position in the structure of the Gospel at one of its major stages of composition, i.e. after the addition of the Prologue which it echoes in the way of inclusio, and before the addition of chapter 21 which follows the previous conclusion, 20:30f). In that sense it is structurally similar to the conclusion of 1 John, houtos estin ho alèthinos theos kai zôè aiônios, "this one is the true God and eternal life".
- I do agree that the legal notion of representative identity (which you have advocated elsewhere, if memory serves: he is God inasmuch as he represents God to the believers) lies in the background, yet I believe that in Johannine theology (contrary to the Synoptics) this notion goes through a particular development where the purely formal or legal aspects give way to more "ontological" and "mystical" ones (as is apparent in the Prologue).
- Last but not least, this Johannine particularism takes place within a (proto-Gnostic) notion of "God" which is both more fluid and more inclusive than later Christian theism (either Trinitarian or Arian), since it extends, through Christ as the Revealer, to the believers themselves: those are ultimately restored to the divine oneness in which they originally belonged (e.g. 1:3f; 6:37ff; 10:3ff, 14f,27ff; 11:52; 17:6,9ff,21ff).