Wrap the barrel with a piece of leather. Clamp it down good. Get a big crescent wrench, the longer the handle the better, and tighten it on the breech plug making sure the end of the wrench jaws to not go over the top of the tang. Tighten the jaws down, then give the end of the wrench handle a smart whack with a rubber mallet. It may take a couple... but as soon as you see the alignment mark go out of whack, you know you've broken it and it will come out from there.
You'll probably have to do the same in reverse to put it back in. That's how I got mine out, no problem. The wrench will mar the breechplug a little bit, but as long as it's below the tang, it will never be seen anyway, as it will be below the wood.