Shift-spacebar brings up the scoreboard, which will show all the allies in blue, hordes in red, and has an icon by their names so you know what class they are. Once the bg has started, you can sort by damage or healing done (or other things like HK's or KB's). That's usually how I identify the healers in bg's.

I was hoping for something a little less massive and obscuring than the scoreboard.

Targeting players in a fixed group will be dependant on them having a unitID.
Party, Raid Members, Arena, Arena Opponents, etc all have those, but not opponents in a battleground, so no what you want is not possible.
Class Coloured Nameplates etc should help once in nameplate range to identify classes, but unless you are going to check for the presence of buffs through powerauras, or as I am sure some can do, show specific ones on the nameplates then you are not going to have any easy way to identify them.
Personally I tend to go by the mana pool, since a high mana pool on a druid if not a moonkin will make it resto, same with paladin making it holy though it is less obvious with a shaman, again the presence of water shield maybe from the previous suggestion.