In beta 3, it was possible to play as a character on the opposing team. When it'd change from FFA to teamplay, a character prompt would pop up. If you cancelled out of it, you'd stay as whoever character you were previously playing as, regardless of teams.
As of beta 4 and beyond, I don't believe this is possible. I have however seen some players being represented as one character, then another when they are killed. Not sure what causes that.