[Dawn]

You can do this with a Tag.

local MAELSTROM_WEAPON = GetSpellInfo(53817)
oUF.Tags["maelstrom"] = function(unit)
	if unit == "player" then
		local name, _, icon, count = UnitBuff("player", MAELSTROM_WEAPON)
		return name and count
	end
end	
oUF.TagEvents["maelstrom"] = "UNIT_AURA"

You can check for player's class in your layout to only "activate" it for shamans.

local _, playerClass = UnitClass('player')

if playerClass == "SHAMAN" then
blubb
end