Anything that requires Clan War/ Tourney moderators to spend time calculating values for each game is too much. Unless you're volunteering
Keep in mind as well that the players playing need to just as easily see if they are in danger, so your calculations would need to be easily accessible to all players.
So basically, something like this would best be implemented on the Massive Assault developer end, but they are (rightly) focusing heavily on releasing MA:PR.
As far as the Chess Clock, I'm not seeing any difference between what you're suggesting and what a Chess Clock can do: Set the Chess Clock to two weeks per player as MortonHQ said, and you have 4 weeks to finish the game, period.
But neither of these methods account for when a game goes extra long (I've had games with other players go as high as 40+ turns). In a tournament, that game would have to go into adjucation as all games needed to be concluded within the 4 week window, but in a Clan War or friendly game environment, you can set a Chess clock to have an increment per turn, say 1 day. So each turn you play then adds 1 day to the total time in which you need to complete the game. Your timer would start decrementing as soon as your opponent plays their turn.
A 3rd setting is to use just the increment per turn, and you have the current Massive Assault setup (0 total time 3 day increment is the same as a 3 day hard limit, as you get 3 days added to your clock each time you complete a turn).
With a variable Total game time and a variable per turn increment, you can come up with a range of possible timing schemes that allow both flexibility while forcing the game to continue at a good speed.