Hey,
when you have missed the FoS for Onyxia's Lair (Level 60), here is your chance to do it today!
You have to go to Nefarian in BWL and loot a t2 helm and for looting this helm you will get your Onyxia's Lair Feat of Strength.
I can't provide any screenshot for this right now but it's working and I'll try post a screenshot from armory later...