I would sugges to use Active Combat Tips system for this task
After you set to 'Always Show' in-game active combat tips, you can utilize fully the event
Code:
EVENT_DISPLAY_ACTIVE_COMBAT_TIP (integer eventCode, integer activeCombatTipId)
Take a look into CombatCloud or LuiExtended for how it is done. Of course, if you do not want to look into default UI tip visual, just hide it manually in your addon, while leaving events system in place.