There are actually four opportunities to kill her SASO without mission stories, two of which are on opposite sides of the top floor. Aside from the veranda like you mentioned, you can actually get on the roof and loop around to the bathroom window that is attached to her office. From there, it's just a game of patience where you lure her into it when she patrols into the office to look for the combination code for the safe. The third opportunity is that she immediately heads to the lab when the virus is destroyed. I don't recall how closely her guards follow her but she does cross underneath the path of a stalactite that you can drop for an accident kill. The fourth opportunity is to grab the prototype virus in storage room attached to Silvio's bedroom, which you can reach by climbing down from the attic. The prototype virus is targeted for just Desantis's DNA, so you can lob it out of line of sight next to her and it will only kill her, which counts as an unnoticed poison kill.
If you're not going full autismo with it, pokes her head out across that whole top floor and there's 3 easy spots you can pop her from (the derelict tower that Pippa did it from, the Clocktower, and Rocco's apartment when she's heading into the office).