The big problem is that a mountainous area does not typically have the resources to support more than a handful of large predators, and the ones that are in such an area are known for long periods of inactivity between feedings (I'm thinking bears particularly).
I'd propose two solutions to this. First is in regard to the scarcety of prey animals in mountain regions. In the most polar areas and much beyond the treeline you are correct, food can be scarce for larger animals. However, if Ogres are not living primarily in those areas that is not a problem... if ogres are as migratory as some of our recent theories seem to suggest, ogres would actually spend most of their time in lower elevations. In fact the only time they would travel above that point would be after they are well fed to return to the 'home camp'. While mountain peaks may be bad for prey animals, just a bit lower on the elevation is great hunting lands in terms of deer, elk, yak, and in our case rhinoxen. Ogres are not living on mountain tops, that is where Yhetee live (as for what Yhetee eat... that is another topic). Also we can't forget that the ecosystem is significantly different than any earth-based ecosystem in that there are huge expanses of tunnels under every damn place in the world it seems... tunnels are a whole new food source to exploit, if not by the ogres than indirectly by the ogre's prey animals.
The other solution is just as you said, periods of inactivity. There isn't much of a 'season' system from what I see on the Mts. of Mourn... just cold and colder... so a ursine hibernation system isn't the key. Look at (I hate to keep going back to the same examples over and over but it fits) seals or walrus pods. Big multi-week hunting excursions, followed by days and days of basking in the sun doing alot of nothing. Of course seals only do this during mating and child rearing season primarily, but there is no reason to expect a species can't make a full time living exploiting such an ecological niche.
Instead of an ecosystem where the big things prey on the little things who eat the littler things and/or plants, we have one where everything is carnivorous and massive. Or a Gnoblar.
I have to believe, then, that Rhinoxes and their ilk are not nearly as common as GW seems to imply and that large numbers of mountain deer, goats, rabbits, fish, etc. exist for the Ogres to feed upon.
So true, the picture we have of the mountains is painfully inadequate to look at it the way we are trying to. Of course there are a huge number of other food sources out there, we just don't know anything about them. But even if we confine ourself to what we know I don't think we are that doomed. Look at Yak: big herd animals traveling in fairly large family groups, often traveling through expanses with very little food between feeding grounds. If we follow that model for Rhinoxen behavior things arn't that bad. There also has to be some sort of high-altitude cold weather vegitation out there without a real-world analogy too it to help bolster the ecosystem... Sky Titans were 'farming' mammoth way up there on something, probably the same somthing (or similar somethings) that Rhinoxen eat.
Furthermore, I have trouble believing that Ogre bread is made solely with the ground bones of their prey. We know that Ogres can ferment alcohol, which implies at least some understanding of grain. Therefore, I can see them sending out Gnoblars to scavenge buckwheat or the like to cook with.
You can make alchohol without grain at all... you can ferment fruit, milk, animal urine, blood... pretty much any damn thing that yeast grows on, and yeast is a very opportunistic little bugger! I'm very skeptical about the idea of ogre bread being from bones... not because of it's inadequacy as a nutrition source but because I don't see ogres as baking. Plus the only time we hear about ogres making bread is from a bull who laughs after he says it, like its some kind of ogre joke that is funny that lowlanders belive it at all.
I think you hit the mark fairly well though. Before we take fluff and massage it into a good, realistic dietary system for ogre-kind, we need to take fluff and massage it into a good, relaistic ecosystem of the environment they live in. Food chains, species lists, that sort of thing. It would be a big endevor, rife with speculation, but it is what the groundwork would be.
Some stuff to concider before we start any of this though: it has to be an ecosystem that can support ogres, but
without them being integral. In ecosystem development terms, ogres are a recent addition to the scene... I'm sure they drove a few prey species and fellow predators into extinction. In fact if we do say that there are very few species in the Mountains of Mourn, it is probably a recent development in response to the devestation caused by the recent (in geological/ecological terms) ogre migration! The only species that we see may be those that responded well to an ogre-full environment.
Alright... Luck Gnobler Tastey: ... Check!
Sword Gnoblers Poke and Prod: .... Check!
With the vital jobs done, everyone else is 'snack gnobler'...