Paizo's Ultimate Combat for Pathfinder presented several rules variants. Among them was the Armor as Damage Reduction rules variant. My attempts to play with this make it look like this will require more than just Data entry to work, and will need a House Rule support option for the game mode.
In addition to support for properly calculating the mechanics, character sheets would need an adjustment to display the information. The change from AC to Defense, and the addition of the Critical Defense Check bonus would both be important.
Ideally, this should be compatible with the other rules variants presented, such as the Wounds and Vigor variant rule.