You don't necessarily need mitigation so long as you make sure the HoTs of the AST are designed around the incoming damage of tank busters and the like. A large volume, short duration HoT can easily restore damage being taken long enough for a WHM to get a heal off. A reactionary/time delay heal could be cast before the tank buster to heal some of that damage back immediately after taking it keeping the tank alive long enough for the WHM to get a heal off. As long as you can keep the tanks alive long enough for the big heals from the WHM, you don't really need mitigation when you have a WHM/AST combo.
If you've ever played Everquest, the Celestial Elixir line of heals is the idea of AST heals I'm having. In FFXIV terms perhpas you would have (and please keep in mind this is a hypothetical example just to give an idea, the numbers would have to be worked out by the dev team, etc) Benediction I heal about 1/2 the amount of a Cure every tick for 3 ticks. Thus, it heals more (50% more in this example) but it takes longer for that heal to be applied.