Sounds like a good outfit for a golem. 0 encumberance and all.
For the abovementioned plan, this won't work because of Golems' lifelessness (and lack of stealth, which the originally-suggested elephants also suffer from). Otherwise, yep, it's a cute trick and can help with niche Golem thugging.
But Trolls have regenerate.
For the abovementioned trick, a Troll King is nonviable due to stealthlessness. Even if he was not, it's a poor choice due to the unit not being 0 Enc since the
Black Heart adds a +5 Enc chest wound...
If you really really really want a trampling assassin, go with a
Lord of the Night with
Boots of the Behemoth. Built-in flight and assassination, with an added bonus of probably having enough Death to cast Soul Vortex to shake off the trampling fatigue. Admittedly, using your pretender in this role is almost certainly a misallocation of resources, so...
...we're left with, hmm...
...Argatha dropping GoR on an
Umbral and giving it the Heart and Wings, maybe? Probably still not the best use of resources, nor an option for any other nation, but at size 4 it's bigger than a Troll King, has 0 Enc, is ethereal, and also has a nice 22 Strength if it runs into something its own size.
[edit] The big problem with this is that a trampler wants to be as big as possible, and there aren't many big stealthy units. If you can settle for size 3 units to weakly trample human-size victims (and ride roughshod over Hoburg commanders), there are a number of options available, but this is very much an instance where bigger is better. Even the humble Jotun Scout could be made into a more effective and damaging trampling assassin than a size 3 unit (though the encumbrance hit from the Heart would make it quite easily winded).