Last night I was on my second run through the story and I got to the predator section at the top of Wonder Tower where Strange is.
Once I was up on the exterior gargoyles I found that if one of the guard is out on the balcony within range of a drop attack, you can hang and the option appears for a inverted takedown. Batman then warps sideways, grabs the guard and pulls him diagonally back to the gargoyle. You can then batarang his rope and drop him, I tried this several times with other gargoyles and the results were sporadic, but they happened. It's the only time the game has let me do something lethal.
Has anyone tried this or has found another way to subvert Batman's desire not to kill dudes?