1. A character Aspect.
2. A temporary Aspect, created through a spell or a mundane maneuver.
I'm trying to come up with something that doesn't require you to spend a load of fate points, so aspects are out.
3. Extra consequences from a stunt like No Pain No Gain.
Good idea. What kind of extra consequences (as in mild/moderate/severe, and should there be multiples)
4. Less harmful consequences from a stunt like Unshakable.
I don't think I've seen this stunt. Would you be able to either post it here or direct me to where I can find it?
5. The Feel No Pain custom Power.
This power explicitly says you "suffer no stress from attacks based on pain." This seemed overpowered to me, since most of the damage that I understand stress to represent (bruises, minor cuts) would, in my opinion, qualify as attacks based on pain. After all, if they're not hurting, they rarely interfere with your actions. Also, what "attacks based on pain" means is fairly unclear to me. I tend to think of it as "inflicting damage that only affects you if it hurts" but I'm not confident about that.
It also offers an additional minor physical consequence, which is a good idea.
6. A stunt letting you use Conviction or Discipline for your physical stress track.
This would be a cool idea, but it's not what this power is narratively. This seems more like what the Winter Knight mantle allows you to do.
7. Inhuman Toughness with an appropriate Catch.
It really specifically isn't this. Every time Harry uses it, he says that it puts him at risk, because not feeling pain is dangerous. Inhuman toughness isn't this at all.