It's not possible; Blizzard has intentionally declined to make this functionality available.
Mods can already assign "mark" targets at will, without even needing user input to do so. If we could target marks, it's a short step to mods being able to select the targets for us, which the UI is designed not to allow us to do.
I play a shadow priest, and I'd prefer to have timers for my MB and SW:D cooldowns in the same bar display as my dot durations. It's equally important to me whether I have a cooldown coming up in the next 3 seconds, as it is if I have a dot wearing off in the next 3 seconds.
I've considered writing this myself as a first mod, but I don't want to reinvent the wheel if there's a mod it could easily be added to. (I also immediately ran into confusion that some Ace mods use CandyBar and others manually code CandyBar-like functionality, but I don't know why. Also, Ace3 doesn't seem to have bucketed events, and reacting to player auras is definitely something I'd want to bucket...)