I cant quite see what you speak of as reasonable, at least as is. However, a secondary single-target, cover-like buff that moderatly-substantually reduces damage taken while behind the paladin could definitly be useful without leaning towards the "OP", especially for front-line melee classes (would work wonders for DRG since they have the mid-range advantage... or at least I still believe that's the case). And just to add more strategy, lets place at minimum a 1 minute recast, so that one cannot simply switch party members on a whim. The buff would last until toggled once more.