deal damage to item in player inventory

I basically just want to use this procedure block for the player's inventory: "Deal {Number} damage to item in slot {Number} of block at x: {x} y: {y} z: {z} if it has inventory" 
is there any way to do this?
i want to use this for my jetpack mod, the jetfuel must be in off-hand to be damaged

(any alternative for doing something like this is also VERY appreciated)