I don't see why you couldn't use it as a foliar spray...
Just found this (looking for "hydroponic" kelp...seems to be all the same thing)
"Foliar spraying is the most effective way of applying kelp, since leaves are up to 8 times more efficient in taking in of nutrients than through the root system. When foliar spraying try to apply in the early morning when the plant is the most active and the stomata are open to their fullest, avoid spraying before it rains, use high quality water with a pH of 6.0, and any foliar spray benefits from a non ionic wetting agent such as Mega Wet."
http://www.simplyhydro.com/benefits_of_kelp.htm
Maybe not the early morning spraying bit though...