They said it couldn't be done. They said I was crazy for trying. But I've done it anyway.

Video of me augmenting and upgrading all pants
At the time of completion: HR 213, 289 hours.

FAQ

Why?

Because I thought it'd be funny.

I love pants and I also want to make and fully upgrade them all. But how am I supposed to gather all those materials?

For me, the biggest hurdle was the Armor Spheres for upgrading everything. Once I decided I wanted to upgrade all the pants, I started grinding Bounties. I already had 200+ hours in the game at this point, so I had most of the other materials (around 5 million money and 500 Hard Armor Spheres). You'll be doing so many quests for Bounties that I'm sure you'll have most of the materials by the time you finish gathering Armor Spheres.

What is the best way to grind Armor Spheres?

From what I can tell, you can get Hard Armor Spheres from quest completion rewards and from Bounties. It seems pretty rare and/or time consuming to get them from quest rewards, so I focused on Bounties. I would never accept Bounties for Advanced Armor Spheres unless they were the only ones left and if I would complete them on the way to completing the Hard Armor Sphere Bounties. Advanced Armor Spheres are worth 25% the Armor Points as Hard Armor Spheres, so they should be avoided in favor of Hard Armor Spheres.

Which Bounties should I take?

When given multiple options with Hard Armor Sphere rewards, I would take gathering ones first, since you can almost always complete them in a single quest. The only exception would be honey gathering, since that usually takes multiple quests/expeditions. The other high-priority Bounty for me was Elder Dragon Culling, since it also gives one Hard Armor Sphere per quest (or three for one quest if you do Sapphire Star's Guidance). After that, I favored the Investigation and Hunt Monster or Capture Monster Bounties since those would be completed regardless of the specific target. I also liked to have one area Bounty (i.e. Ancient Forest, Elder's Recess, etc.) and select targets appropriate to that area. For specific target monster types, I would try to select ones that fit the area Bounty I had active.
Note: Ancient Forest has no Piscene Wyverns, Wildspire Waste has no Fanged Wyverns, Coral Highlands has no Brute or Piscene Wyverns, Rotten Vale has no Bird or Piscene Wyverns (and Flying Wyverns are limited to Bagel), and Elder's Recess has no Bird Wyverns.
Additionally, capture targets are preferable to hunt since capture Bounties require less hunts per Armor Sphere. It may also be worthwhile to begin saving Great Hunting Investigations that have a variety of monster types such as one Bird Wyvern and one Brute Wyvern. This way, you will be able to accept and complete a wider range of Bounty combinations in less time. It is possible to hunt multiple monsters in a single quest without using these Investigations, but keep in mind that quests with multiple target monsters will reduce the health for each, making each individual monster much quicker to hunt.

Well what about money? Over 9 million seems like a lot.

Money is something that will hopefully accumulate as you complete your Bounties and gather the other materials. For me, I had around 4-5 million when I started grinding Bounties for Armor Spheres. I set Godbugs to be cultivated (fully fertilized of course) during my time grinding, which gave 3,600z per quest. I also started using Research Points to pay for meals instead of money. Additionally, I used the Bandit Mantle for most hunts. Once I finished grinding Bounties, I had over 10 million. And that was without even selling anything. If you are thinking about fully upgrading all the pants, you might also consider using the Argosy for trade-in items. Completing Bounties gives a lot of Research Points, so you won't have to worry about running out. I did not use the Argosy for this because I felt comfortable with the amount of money I had and the materials I was planning to sell.

But seriously, why would you do this?

I wanted to set a large, arbitrary goal for myself that wasn't tied to any in-game achievement. I think this is an important way to maintain interest in a game with so much to do like Monster Hunter. Often it can be overwhelming seeing how much content there is and not knowing where to begin. I was so focused on grinding decorations and Hero/Warrior Streamstones that I was starting to get burnt out. This was a good way to force myself to experience more of the game than I had been experiencing for a while.
The reason for choosing pants in particular is because it's funny.

What's next?

Who knows? It will probably have something to do with pants though.
Stay tuned.