I'm sorry, but the idea of LFR players being gipped out of the full Wrathion questline if the legendary doesn't drop in LFR is bogus. You are not entitled to all the content without meeting the bar for effort put forth. And the effort put forth in raiding LFR, even if you do every wing, every week, is microscopic compared to the effort put forth in just raiding normal modes.
You are not entitled to the 150 mount achievement mount if you only have 5 mounts. You are not entitled to the Operation: Shieldwall storyline if you only roll Horde. You are not entitled to the meta mounts from the holiday achievements if you only did that one boss during the summer festival.
Legendaries are like those things - they require a certain amount of effort. LFR nets you some of that effort, and I'm willing to bet, likely most of the effort needed (where most of the effort merely involves time). But it is nowhere near all.
Oh, and it doesn't have to be a 1 at a time thing, Tehstool. Just have drops of some reagant in normal/heroic that everyone can loot, like this first tier, and then some version of a Sha-touched weapon off Garrosh which combines to make it legendary. IE, epic gun off Garrosh, I've farmed all the shit I need to in SoO, combine to form legendary gun. In that way, the legendary is a result both of a long grind (a la Wrath/Cataclysm) and a bit of luck (getting the prereq epic to drop off a SoO normal/heroic boss), a la Vanilla/TBC.