Hey, kreynolds,
Having finally had a chance to properly digest your homebrew rules, I had a few minor comments/questions:
1) I wonder if Natural AC is the best choice. It seems a little awkward to me. Natural AC tends to be more "personal rigidity" than a suit of armor. Perhaps just calling it a Hardness Bonus, or some such? Plus, that way it can still stack with another source of actual Natural AC.
2) Maybe I'm just a cronic tweeker, but if I had my druthers, I'd maybe alter your numbers a bit. For armor, perhaps simply double the "+" of the DR to get the additional hardness bonus. So 5/+1 and 10/+1 both give +2 hardness. 15/+2 gives +4 and 20/+3 gives +6. Shields could be half that (or equal to the DR "+", in other words). I would also think about allowing dragon hide armor younger than those with a DR rating to get a flat +1 hardness (none for shields), simply because it is still dragon armor, even if fairly young. After all, many dragons can be size Large and still not have DR.
3) Why not allow scale mail armor? That seems like something feasible.
4) In our own rules set, we relaxed the size requirements a little as well. Maybe you should look into that. As it stands in the MM, in order to make medium-size fullplate armor, the dragon must have been Colossal in size. When you realize that only Red, Gold and Silver dragons ever get that big, it can be rather restricting and silly. To us anyway, but YMMV.
Interesting idea, all-in-all. Good job.