If you want to make armor add bonus to A_D's proposed solution to fortification I suggest having the armor add it's enhancement bonus to armor class against crits.
For example:
+5 full plate = 15 extra ac vs crits (x3 the enhancement bonus)
+5 breastplate = 10 extra ac vs crits (x2)
+5 chain shirt = 5 extra ac vs crits (x1)
Robes = 0 extra ac vs crits, or possibly half of the armor bracers bonus so +4 extra ac vs crits for 8 bracers, +3 for 6-7, etc.
Shield could do this as well, light shields = x1, large shields = x2, tower shields = x3
if you have a shield bonus from another source than a shield you could give half of it to the extra ac vs crits.
I am unsure on the warforged though, perhaps composite treated as light armor, mithril as medium and adamantine as heavy while the reinforced plating either adds +2 to the enhancement bonus before armor type is taken into account.