Unless you are grouping with the same people every time this would be extremely unfair.

A more fair solution, in my opinion, would be to offer a guaranteed drop from the content that can be used as currency to buy the mount. For example, you get a guaranteed "fire orb" from Ifrit EX and when you have 30 orbs you can trade them in for the mount provided you don't already have it. The quantity required is just an example, it could be anything that SE wants it to be, but at least you know that your efforts aren't for nothing.