It is certainly possible, just takes a bit of coordination if you are playing a team based match there. You jump from the corner of the building pointing to the corner of the downed scaffolding. It is kind of obvious to experienced players when someone is trying to get up there, so you need your teammates to engage at all points from the middle of playing field into the enemy side. Once up top, an OMA or scavenger pro setup works well with sniper rifles or OMA and grenade launchers. Tactical insertion is a must if you are going to go to all of that trouble, but the spot is worth it if you're good enough. I thought everyone knew you could get up there, but since so many were unaware here is another spot you can get into on highrise:

On the building opposite of the one with the downed scaffolding you can get into the ceiling by climbing on to the yellow crane on the middle of the field opposite of the helicopter pad. In fact, if you want to you can get onto the crane from the helicopter pad, or you can climb the ladder near it and jump on to the pallet of I-beams. Alternatively, you can get into the scaffolding below the I-beam pallet and jump up to the pallet, you just have to make sure you keep hitting your jump button to pull yourself up. Once you're on the crane you can either head to the side with a dead-end and set up a nice sniping spot, or head to the other end to get on the ledge leading into the building.

Protip part 2: You can jump on to the metal rods poking out of the ground at flag B in Karachi. You simply need to get on to the rooftop overlooking B and jump out to the one nearest the highest rooftop there. Marathon and Lightweight are necessary, but once you make it on to the first one, you simply jump to the 2 nearest until you can jump into the 3rd story ledge at B. From there you jump to the ledge next to that one until you are lined up with the rods nearest the half destroyed building that acts as a shortcut between A and B. Jump onto the first one of those and then you can land in the "V" shape above the hole overlooking B above the staircase in that building. If you did it right, you will land above the hole and can now walk around outside the map. Just be careful because some of the areas that you can reach are not developed and you can fall through the map. Doing this isn't really feasible in a live game because the quickest I've ever seen this done takes roughly 2 minutes using tactical insertions, but it is neat because you can walk for quite some time in the map.