Yes you can use LK as a foliar spray, I used to do this... I think it was like 2.5 or 5 ml per gallon or what? Two teaspoons per gallon for soil applications. The usage rates should be right on the bottle, including for foliar.
It isn't a pesticide or anything though. If you have insects or your leaves I would say at least add a couple drops of Dr. Bronners' Peppermint soap.