Why dont limit the time for capping for each player?
I mean: 15 minutes is the duration of the match: u have to attack for 7:30, and 7:30 to defend. To do this it should be implemented that during defending time, the flag can't be taken. The time can be splited in other ways: 2 mins attacking, 2 mins defending and so on for the rest of the game.
The server could decide at the begginning of the match the players that can attack and the players that dont, splitting the single team in attackers and defenders and change them during the game (a countdown can be shown for remaining time in assigned role).
What do u all think about this idea?