The wording of Hurl Through Hell suggests that the damage is taken when the spell ends.
At the end of your next turn, the target returns to the space it previously occupied, or the nearest unoccupied space. If the target is not a fiend, it takes 10d10 psychic damage as it reels from its horrific experience.
I read it as "the target returns, then takes damage as the trauma overwhelms it". From a role-playing perspective, you could argue that the little roadtrip through hell it took is such a supernatural experience that the target can't even perceive it properly until it returns back to its familiar plane.
Should you choose to accept this reasoning, then the damage roll would succumb to the same rules as the other rolls.
However, such ruling is not immediately obvious, and the wording on Hurl Through Hell is still ambiguous enough that the players and/or the DM might feel more comfortable imagining the damage taking place in Hell. In that case the specific rules for Mechanus would be avoided. In the end, it is up to the DM.