PLD if I remember right actually gets a trait somewhere between 60-70 that halves their spell costs and also prevents interrupt from damage. This is why they can cast clemency and holy spirit at 70 without getting interrupted when hit.

As for a caster based tank the real issue I see is that they would need a trait that allowed casting while moving unless they plan to eat every AoE to the face. I think a better option would be a tank that dealt magic damage via melee attacks and weapon skills.