The only way for such a mode to work is if both teams have to do the same thing, ie one player has to stand on the spot of the opposing teams bomb for a set amount of time, this would reset the timer, if this player is killed the timer would restart. The winners are the team that prevent their bomb going off.
Edit: Actually i guess this would end up a draw most of the time :(
Edit: Actually i guess this would end up a draw most of the time :(