Dungeon Fantasy Monsters 3: Born of Myth & Magic, contains a new meta-trait that I wrote up.*
So, where to apply it?
In my games, it's applied much more broadly than just to Rock Trolls. In general, I give it to rock and stone types who I feel should be more worried about crushing damage that pulverizes them than cutting damage that slices them.
Here is the general breakdown I use for Homogenous beings:
Stone: Amorphous Stone
Water, Slime, etc.: Homogenous
In other words, just stony, rocky beings. Not all of them, either. If I feel like their main weak point should be shattering, cracking, being broken to powder, etc. I give it to them.
Rock trolls have the Amorphous Stone meta-trait. So do stone golems. Obsidian golems have it as well. So do Obsidian jaguars.
But strange creatures like the living pit do not - they're not more likely to shatter, and for all of their rock-like features they aren't really rocks to be broken up. Earth elementals generally don't have this - they are earth, stone, clay, etc. all mixed together; they're more resistant to breakage than vulnerable to it.
If a stone creature write up has Fragile (Brittle) or a Vulnerability to Crushing damage, I'll use the trait for that creature. I'll generally replace the multiplier for Amorphous Stone with a stronger modifier if the creature has one - I won't add or multiple them.
So this is how I always liked to run Homogenous in GURPS 4th edition. In 3rd, I used to put "No Cutting Multiplier" on monsters and give them Vulnerability to crushing damage. In 4th, I basically just ran the trait like it was Amorphous Stone on stone creatures. No longer was a sword the best way to kill people, cut down trees, and shiver stone golems. Crushing damage had a place.
That doesn't mean cutting damage is useless. In actual play, you'll see my games end up with clearly-stone creatures getting waxed by swordsmen but only damaged by crushing attacks. A good part of that is sheer damage. When the heavy fighter with the greatsword does 2x as much damage as the crushing weapon fighters, he's still going to kill many of them.
And I'm a lot harsher on "using the flat of your sword" for crushing damage. When that damage is in the 3d and 4d range, it seems likely to me the sword would suffer from the abuse more than it would make a better hammer than a hammer. So I roll for swords bending on heavy hits. By all means, cuff the hirelings with the flat of your sword . . . but smash stone golems with it at your own risk.
And this is slightly off-topic, but it's worth mentioning to my players - elemental shaping and destroying spells work on animate beings only if those beings possess a special vulnerability to them. Purify Air, Shape Earth, Earth to Air, Destroy Water, etc. - they do nothing except to beings that have a Weakness disadvantage that points them out. The spells may have a use against them but they aren't automatically converted into damaging spells. The caster won't know anything except that it failed to do anything - this is worth research and Recognition can help. After all, some beings are straight-up weak against those spells. But just because something is or could be Amorphous Stone doesn't meant Earth to Air is a damaging attack spell. It probably will just fail; save it for the inanimate rocks you face.
* With critically important support from Sean Punch.