There's a reason why a lot of Kyokushin schools won't allow face contact with the fists in their full contact competitions / sparring.
There's simply no safe way to "condition" your face, unless the goal is to destroy all of those nerve endings. Unless you're a boxer who has been pounded many times in the face, and have no live nerve endings there, your eyes will swell up in tears (nothing unmanly about it, mind you), and your nose will swell, when you get stuck in the face.
While it may seem to one's advantage to dull those endings, I strongly recommend against it, since you'll end up paying for it when you're older. Quivering lips, shaking jaws, etc., are something I'd rather not see amongst any Karate-ka.
Regarding head contact, though, I've watched a good number of Oyama (Yasuhiko) Shihan's competitions here in the Birmingham area, and while they don't allow hand contact to the head, they do allow kicks to the head, as well as the thighs. Thus, in their school, I'm quite sure that their guys are well aware of guarding the head, since those head kicks do hurt...
Talk to the instructor, and ask what kind of sparring do they do, since not all Kyokushin schools will be alike.