'tain't that hard to model with mechanics, James: when the troll is crit with a slashing weapon, it spawns a TROLL LIMB in an adjacent square. Troll limbs have 1 hp, 10's for their defenses, and attack like the troll, but at disadvantage and dealing -4 damage (or whatever). Heck, it'd even be EASY to do in 4e, and make trolls an interesting Elite!
So, I don't buy the line he's feeding us there.If you WANT to do it, it's doable.
Personally, I think it's smart to do it. Trolls in D&D make good "elite"-level monsters. Things like regeneration and severed limbs that attack make them worthy of that XP total. This means that even at their normal level, trolls are scary things. Which works really well.
Possibly, they don't WANT to do it. And it's probably fine if they don't. I don't think it'll break any hearts. I think the possibility of doing it is a little more interesting, and deserves to be explored, but if they want it to be only color, it can be only color. The question being, would anyone who didn't narrate the trolls this way be missing some essential part of using a D&D troll? And I don't really think so.
It's poisoned fruit, isn't it.
A trap!
Oh no. They'd never but a complicated mechanic for severed arm in the troll entry. It'd be either too short and be extremely disassociated and filled with holes. Or extremely long paragraph or cumbersome mechanics.
But with the seed planted, foolish DMs will attempt it on their own... only to have their games bog down into a dragging oblivion of contradictions, confusion, and missing pieces.
Bravo.
Here are your 3 Evil points, Mr Wyatt. You have earned them.