This WeakAura will alert you to equip your Onyxia Cload when inside Nefarian's Lair and alert you when you have your Onyxia Cloak equipped outside of BWL.
This WeakAura also comes with 2 options, one called FR that if marked will alert you to equip your cloak when targeting the 3 drake bosses in BWL and another option called Ignore to ignore all alerts.