I haven't used JBweld in that type of repair, but something called fireseal2000 from POR15...it's more of a ceramic paste that is made for manifold repairs. It's held for a few thousand miles on my series truck exhaust manifold.
Now, since it worked for me, I tried it on my Civic when that exhaust manifold/cat converter/all in one/nightmare cracked, but that crack (about 1" long and 1/4" wide when I cleaned it up) was way too big...so it's fine for small cracks...
Badger - GO FOR IT! That stuff is "DA BOMB"...I have not used it personally (thankfully) for that or things like it. BUT...on our ROAV Spring Fling a fella in a Series truck put a hole in his transmission. I gave him my tubes and he bonded it, drove all the way from Sthn VA to home in Nthn PA without loosing a drop....I have heard MANY stories like that so it will fix it for sure...just be aware of curing times and do not try it on a hot manifold.