I believe that making event quests/adventures limited gem items might be going too far as it requires players to repeatedly make purchases during an event. However, making an event have a gem buy-in to simply get started or beyond a very early gateway might be a viable way to make money.
However, doing something like this opens a certain pandora's box in that each and every player who pays to participate in this manner will likely have a higher expectation of the event itself. Changes midstream, failed removal scripts, etc are things that would be highly flammable topics when the entire participating population in an event is composed of those who wanted it badly enough to pay for it. And rightly so.
With adequate planning and good execution I'd be in favor of such a model.

