Well, can Dark Charm be used to move heroes off the map in general? Say there's a battle at the end of the map where heroes may exit and you just want to get rid of one. What's the general rulling of that as long as they are within the movement point range required to do so? The answer to this more basic question would inform the more advanced one.
I'd be inclined to say yes, in that case the OL could DC move the hero off the map. However, that doesn't really change anything because this new hypothetical quest rule is one that applies to heroes, while the Masquerade Ball quest rule applies to monsters. It doesn't change anything.
My group was playing "pilgrimage" for the interlude and I wanted to attack a golem with dark charmed hero. The group unanimously stated that the "heroes can't damage the golem" applied to the scenario as stated in the rules and I caved to the pressure and let them have the easiest win there is.
I'd say you could DC attack the Golem in this case, since per my proposal above, the hero would be counted as a monster for attacks. Regarding tokens "treated as heroes" the purpose of having them be treated as heroes for attacks is because monsters are only legally allowed to target "heroes" with their attacks. A similar logic applies here.
Having since researched this I've found that the hero is treated as a monster for the single attack or single movement. So much so that even Leoric's -1 damage aura would take away a damage point from his own attack and the charmed hero can't move through other heroes but can move through monsters.
Leoric's ability is a hero ability, hence the DC'd hero counts as a monster and gets dinged (even if it's Leoric himself who has been DC'd.) The movement thing is a good point, though. I don't know that it necessarily implies anything relevant to the Masquerade Ball example, but it's worth taking note of.
Again, this is just how I'd treat it personally. We have yet to hear FFG's official answer, but it's probably something that should be officially addressed once ad for all - what does "treated as a monster" mean, and how is it different from "treated as a hero," if at all?