sorry for the delay in getting back to you on this.
given the restrictions Blizzard places on querying for buffs/debuffs (i.e. querying requires a unitid), this would really only be possible for the player's current target.
and even assuming that problem could be solved, there would be the requirement to identify nameplates and associate them with units. Blizzard doesn't provide any means of doing that either. only the nameplate for player's current target can be positively identified, based on its alpha settings. nameplates with raid target assignments (icons) can also be identified, but that would require every such target to be marked (impractical in large non-elite trash mobs).
anyway, between these two limitations, unfortunately it becomes basically impossible to provide any sort of reliable indicator like this.