I'd say that they could tweak the regular legendary drop rate to make room for these context-sensitive things, but then it would affect less-skilled players adversely. Seriously, though, I don't consider myself a very skilled player, but I could easily get lucky with a couple of these things every once in a while, and it would feel awesome. Even if you pull it off and nothing drops, they could have a little prompt pop up on the side of the screen, a little nickname for whatever context-sensitive action you performed. Maybe a little xp or glimmer boost.