WowAce

Title says it all, here's my implementation for this feature.

It probably could need some improvement as it's pretty dirty code, but it gets the job done.

-- Separate new action highlight cache
lib.NEW_ACTION_HIGHLIGHT_MARKS = {}

function ClearNewActionHighlight(action, preventIdenticalActionsFromClearing, value)
	if action then
		lib.ACTION_HIGHLIGHT_MARKS[action] = value
		lib.NEW_ACTION_HIGHLIGHT_MARKS[action] = nil
	end

	for button in next, ButtonRegistry do
		if button._state_type == "action" and (action == tonumber(button._state_action) or action == button.SpellID) then
			UpdateNewAction(button)
		end
	end
	
	if preventIdenticalActionsFromClearing or not action then
		return
	end

	-- iterate through actions and unmark all that are the same type
	local unmarkedType, unmarkedID = GetActionInfo(action)
	for actionKey, markValue in pairs(lib.ACTION_HIGHLIGHT_MARKS) do
		if markValue then
			local actionType, actionID = GetActionInfo(actionKey)
			if actionType == unmarkedType and actionID == unmarkedID then
				ClearNewActionHighlight(actionKey, true, value)
			end
		end
	end
end

local function MarkNewAction(action)
	lib.ACTION_HIGHLIGHT_MARKS[action] = true
	-- New Actions are cached separately to avoid accidentally overriding them
	lib.NEW_ACTION_HIGHLIGHT_MARKS[action] = true
	
	for button in next, ButtonRegistry do
		if button._state_type == "action" and action == tonumber(button._state_action) then
			UpdateNewAction(button)
		end
	end
end

-- Highlights Actionbuttons by Spell ID
-- Also checks if the button spell is an action with the same name. This is for spells that have different IDs, but essentially are the same.
local function MarkSpell(spellID)
	lib.ACTION_HIGHLIGHT_MARKS[spellID] = true
	
	local SpellName = GetSpellInfo(spellID)
	local ButtonSpellName, ButtonSpellID
	for button in next, ButtonRegistry do
		ButtonSpellID = button:GetSpellId()
		ButtonSpellName = GetSpellInfo(ButtonSpellID)
		
		if Spell and button._state_type == "action" then
			if (spellID == tonumber(ButtonSpellID)) then
				button.SpellID = ButtonSpellID
			elseif SpellName == ButtonSpellName then
				lib.ACTION_HIGHLIGHT_MARKS[spellID] = nil
				lib.ACTION_HIGHLIGHT_MARKS[SpellName] = true
				
				button.SpellName = ButtonSpellName
			end
			
			UpdateNewAction(button)
		else
			button.SpellID = nil
			button.SpellName = nil
		end
	end
end

-- Clears all hovering highlights
local function ClearMarkSpell()
	-- Leave new action highlights alone
	for k,v in pairs(lib.ACTION_HIGHLIGHT_MARKS) do
		if not lib.NEW_ACTION_HIGHLIGHT_MARKS[k] then
			lib.ACTION_HIGHLIGHT_MARKS[k] = nil
		end
	end
	
	for Button in next, ButtonRegistry do
		if Button then
			Button.SpellID = nil
			UpdateNewAction(Button)
		end
	end
end

hooksecurefunc("UpdateOnBarHighlightMarksBySpell", MarkSpell)
hooksecurefunc("ClearOnBarHighlightMarks", ClearMarkSpell)

hooksecurefunc("MarkNewActionHighlight", MarkNewAction)
hooksecurefunc("ClearNewActionHighlight", function(action, preventIdenticalActionsFromClearing)
	ClearNewActionHighlight(action, preventIdenticalActionsFromClearing, nil)
end)

function UpdateNewAction(self)
	-- special handling for "New Action" markers
	if self.NewActionTexture then
		if self._state_type == "action" and (lib.ACTION_HIGHLIGHT_MARKS[self._state_action] or lib.ACTION_HIGHLIGHT_MARKS[self.SpellID] or lib.ACTION_HIGHLIGHT_MARKS[self.SpellName]) then
			self.NewActionTexture:Show()
		else
			self.NewActionTexture:Hide()
		end
	end
end

Implemented independently and simplified for the next alpha version, since there can be only one spell currently being highlighted.