Sustained heat would be a problem, since knives are tempered by baking them in ovens for hours at a time. But I don't think you need to worry about it with transient contact. People have been stirring fires and retrieving kettles and lost forks for centuries using a knife among the coals and there's no harm done.
If you're really worried about this, you could quench the knife in water as soon as the plastic is firm. Or even make a wooden form to bend on so the knife doesn't get hot at all.