The way I see it is, if I can be turned into a human for the duration of an Old Hillsbrad Foothills dungeon or any one of a number of races for scenarios, then there is no reason why the same can't be done for the duration of a BG, in order to balance the teams.
Sure there's always a few idiots who'll complain about having to play for the other faction, but as far as I'm concerned, if they want to be petty about what they play as, then they should just click on 'no, I don't want to play for either team', shut up, and live with a longer queue.
In Rift you play as a mercenary for the other side if the teams are imbalanced, without needing to change your race, in GW2 there's only one faction, and you're kind of playing against a different version of your own faction from another dimension (and while WvWvW can get really imbalanced, having 3 teams means the smallest can always ally themselves with one of the others, or just skirmish while the opposition teams are distraced while fighting each other).
The fact that there are plenty of very easily implemented ways to fix these kinds of problems, without messing with game lore any more than has been done already, and none have been taken, just goes to show how little the WoW devs care about this part of the game.