[AtomicLabMonkey]

A steel or iron plug will be fine in a tapered pipe thread joint.

A straight threaded hole using a regular bolt needs a sealing washer between the part's outer surface and the bolt head. If that washer softens up because of the temperatures involved, then the joint loses preload and will eventually leak and loosen up. All this depends on the specific temperatures involved. Copper crush washers are used routinely on cast iron brake calipers that obviously can get very hot during operation, but a hot caliper is probably still quite a bit cooler than an exhaust manifold. So, I dunno. Might work, might not. Hard to be more specific over the internet.