So, despite the fact that my guild master was yellin at me pretty much all night for doing it, I got to bring my Pally alt for a full Black Temple clearing. I had two justifications for being there. I was giving a normal tank a night off so they don't wind up getting burned out, and the second but better reason is that my alt was the only possible choice for tanking the second flame because nobody else had an appropriate set of FR gear.
Illidan was very stressful, because my guild master was really angry by the end of the night, he didn't want me there on my alt, but as I said, there wasn't any other choice, so I was in. :) The summary: I held aggro, I kept him in range of the melee to DPS, I didn't roast the raid, and I dodged the eye beam. I know that if I failed in any of that I was going to get yelled at like there was no tomorrow. :P
I would have won the shield had it dropped, but sadly, it didn't. However, through the overall BT run, I managed to pick up a tanking belt from Bloodboil, T6 shoulders, and a T6 BP. That's about 1.4k HP worth of upgrade because I've popped epic gems in to them, and most of the pieces they replaced were from Kara.
So, don't let people tell you you can't tank something. There's no such thing as a single-MT guild anymore, and a pally tank has a place in every fight in the game.