Leap attack requires a charge, Momentum swing needs a charge so that the next attack gets a bonus. Is that possible?
Personally, as a DM, I would say no, because you have leaped into the air and landed striking a blow against your enemy, effectively ending your charge, and releasing the momentum you would gain from that charge with your attack on this turn.
I wouldn't personally because my PCs are devillish bastards who would take that and run, stacking 6 or 7 things that require a charge, and citing this incident as a precedent, demanding that it makes sense and would work. Urgh.
However, yeah, it's totally up to the DM.
Honestly this will probably come down to DM's Discretion. From a logical standpoint it makes sense that you could combine the two, but with a penalty to hit on the momentum swing.