Well, I didn't get an answer here so I tried in other locations before the campaign started.
On the Paizo forums I was told that though there is a lot of overland travel in Jade Regent's first three chapters, most of what happens when you might be on your mount would be random encounters and roleplay encounters, and that most bosses and major things are in caves/dungeons where mounted combat would be tough. I wheedled information to the same effect out of my GM.
When the campaign started, I played a samurai (cavalier alternate class from Ultimate Combat), which still has a mount, but was not planning on specifically taking any mounted combat feats so as to not make that my character's primary focus.
The character worked out OK (in fact with his bow he was hell on wheels), but I didn't get to use my mount much in the campaign. Character sheets and session summaries from the campaign for those interested in the details: geek-related.com/session-summaries/jade-regent.
I tried to get some mounted work in early on (and rode right into a goblin pit in the first chapter), but then most of the multiple chapters of caravan travel used a "minigame" kind of combat instead of normal combat so during the time you'd think I'd be charging people/monsters across the taiga, not so much. And of course in the late game as usual it's all dungeons and castles and such.
Since I didn't invest options into mounted combat it didn't hurt to have a horse around occasionally, but having completed the AP I would not recommend anyone use a mount centric build in it.
Towering Mountain of Rage
Right, you want to to propagate Rage effects throughout a tower of creatures. That's... actually kind of interesting. Beast Totem chain grants Pounce and 2 Claw attacks, substantially improving the combat prospects of all creatures in the tower.
Well, the applicable rules are contained here, Pathfinder's Mounted Combat Rules on PFSRD.
Looking them over, we have some... interesting interactions. For a start, as your Half-Orc mount is not a 'combat-trained mount', you have to make a move action to control him in battle as he becomes frightened. Yes, the condition. No, I am not making this up.
But hopefully we could waive that somewhat ill-considered clause (what about an intelligent animal? ugh) and assume a half-orc with player class levels is 'trained for combat riding'. So, you're looking at a DC 5 check, probably with a -5 since a half-orc is not 'suited to riding', to keep the use of both your hands and 'control him with your knees'.
I'm struggling to contain my giggles as I write this.
A slightly more difficult attack is to 'Fight With A Combat Trained Mount', a DC 10 Ride Check, with again that pesky -5 for being ill-suited as a Mount. With Ride as a Class Skill, though, it's probably pretty easy to make this check, especially with an Exotic Military Saddle.
Note that EACH rider would have to make this check, so 3/4 of the stack if you're going Mammoth->Gorilla->Half-Orc->Gnome. I'm not sure if Gorillas get Ride as a class skill. This is particularly problematic as if the Gorilla gets unseated from the Mammoth... well, actually, both the Half-Orc and the Gnome would get to roll to 'Stay in saddle', actually, which is only a DC 5 check. Presumably the Gorilla could attempt to 'Fast Mount' in his next turn, thus returning the stack bonuses to the Mammoth.
I can't find any rules in the Mounted Combat section or the Ride skill to disallow this.
I also think it would be awesome, and utterly approve of the mobile 'Tower of Rage'. Note that the Entire Stack would act on the Initiative of the gnome ("Your mount acts on your initiative count as you direct it. You move at its speed, but the mount uses its action to move.")
I assume this means that the stack acts on the gnome's initiative, but moves at the Mammoth's speed (and if it charges, they all count as charging). Note, I am unsure if a Small Character with 10' of reach from a spear could actually reach the ground if he was on the back of a half-orc on the back of a gorilla on the back of mammoth. I think he couldn't, actually. Ergo, he should probably multiclass to bard and do some form of 'rage music', because, why not.
There is a catch, however; Ferocious Mount mentions that you must spend an extra round of rage to spread the rage to your mount, which the mounts can't do as they have no rage class ability. Greater Ferocious Mount makes no mention of this, but Rage powers don't activate if the wielder is not in a Rage, SO while you can make a Tower of Mounts, you cannot make all those mounts rage unless each mount in the stack is also a Barbarian.
It's worth noting that if each mount is a Barbarian, they will pass on any continuous Rage powers during the Rage to any Barbarians lower in the stack, potentially making the lowest Barbarian have many many many Rage Powers.
But yes. Unfortunately as is it doesn't work unless all but the lowest 'mount' in the stack are all Barbarians with enough Rage rounds to make this worth doing.
Awakened Cat Barbarian riding a Gnome Barbarian riding an Orc Barbarian/Warchanter with war Drums riding a Minotaur Barbarian riding a Huge-Ass War-Mammoth is essentially 90% of the point of playing Dungeons and Dragons.
In 3.5e they would all be wielding monks, as monks are a manufactured weapon.
Probably it would end up looking a little bit like this;
YOUR LIGHTNING IS ALL I NEED
Best Answer
You want to get your mount the universal monster ability Compression, or an equivalent effect. There are several reliable ways of doing this:
1) Choose an appropriate mount: This is actually surprisingly difficult to do, if we limit ourselves to just normal animals listed for use as mounts by medium humanoids the only option is the Dire Weasel, though a Young Ogre Spider would work extremely well for characters with the ability to handle vermin/spiders (and would definitely be worth spending a feat on).
2) Use spells (there's basically only one realistic option here):
3) Use feats
4) Use Magic Items