If you're willing to use Elder Charms in LFR you can also switch spec when the boss dies before using the extra roll. You'll get a Prot roll for the main roll (if you killed the boss as Prot), but the Elder Charm roll will be for whichever spec you're in when you press the button (Ret, in this case).
Is this true or false? Read this on the forums somewhere. I've also heard otherwise.
Need to know if I can kill Tayak as prot, switch to ret before the timer is up and get a chance at the Waistplate of Overwhelming Assault