crazy idea

If an aluminum alloy is OK to use as a head or piston material, I cannot see why it cannot be used for exhaust manifolds. It should not melt. But the strength issue is another problem. You might end up with manifolds 2X as thick as the iron, just for strength. Well, there goes all your weight savings!

A356 alloy is commonly used for heads and blocks. Melts at about 1000°F.