The gullet does have a natural armor bonus. 1/2 normal for the creature, this is where the description states what is allowed for the creature's AC (no Dex, no size, 1/2 natural armor). The fact that no Dex applies to this AC is just a formula to figure the AC for the DM not a denial. Caltrops state, "For this attack, the creature’s shield, armor, and deflection bonuses do not count. If the creature is wearing shoes or other footwear, it gets a +2 armor bonus to AC." This does not mean that the creature is not wearing armor unless it has shoes on. This does not mean the ring of protection +3 does not work, just that AC is figured differently in this case (deflection doesn't help against spikes on the ground).
No one has mentioned that size modifier doesn't count, meaning size does not matter in the gullet. Even though you are surrounded by the gullet and it is thus the only thing you can hit besides yourself, its size doesn't make it easier to hit!
SO, Nail, you would deny the rogue the ability to sneak attack a T-Rex from outside? (Reach vitals?)
And I don't see how being inside a T-Rex is safer than being outside one. Inside damage per round 2d8+8 plus 8 acid = 18-32 per round auto. Outside: 3d6+13 = 16-31 per round if you are the target of his only attack and if he hits and if he successfully grapples you. Given, two of the ifs are pretty much auto, but choice of target is not.
Ciao
Dave