I don't see the need for this, you have the high ground, you already have an advantage. And If you really want to jump down, jump behind your enemy and shoot stab him in the back
Fixed that for ya.
(Even if there is no backstab, just use your tactical-knive or bayonet.)
As a light you can run faster and reach higher places thanks to double jump/wallhop (it's more or less the same)
I think there is no simple double-jump, but it always requires the wall (? Wall-jump). Because in the "Freedom of Movement"-video you can see the player performing a wall-jump and there is a noticeably repel from the wall.
On the OP itself:
I don't think we need this kind of melee-attack, really. This is not Assassin's Creed after all and also it would require a canned animation, which the developers avoid by all means in terms of movement.