Sure but in rugby they are pretty static when they do the lift. Even having the height advantage, with a static target you would need the corner to be incredibly accurate to hit the man. The header would then have no momentum to generate any power on the header, as well as having to direct the header.
Its a nice idea, but even if legal, i dont see how it works.
Sure, it would be a bit tricky to get right, but at worst you miss the man and get a far-post delivery and at best, a free header on the six-yard line.
But is it legal?