I like the general direction in which you've taken the half-orc. However Orc Ferocity feature seems to me to be an ability that will come into play so rarely as to not be useful (only at the very rare 0 hp).
Instead I would suggest a +2 racial bonus on endurance related checks (as the endurance feat grants) and that half-orcs be allowed to sleep in medium armor without becoming fatigued. The half-orc is also considered to have the endurance feat for purposes of prerequisites of other feats.
This increases the chances of them taking the die-hard feat which would be better than the orc ferocity feature.
The question here is: will the Diehard feat remain in PFRPG as is or be modified like some other feats?
One would think that Orc Ferocity points towards the latter developement.
I think you are misunderstanding Orc Ferocity. It doesn't only function at 0hp, but at -1 through -9 HP; essentially, all Half-Orcs have a limited form of the Diehard feat. They're Disabled at 0 through -9 HP, but only for one round. After that, if they're not at 0 HP or above, they're dying.
Eeep! You were right I did misread it. That makes Orc Ferocity a much better feature. That being said I'd still like the addition of the above mentioned benefit.
Good idea.
You might want to check out my Half Orc Thread.