I'd like this too, especially with secondary and tirtiary time controls, ie 40 moves in 40 days followed by 20 moves in 20 days followed by 10 moves in 10 days (repeating).
Would add some interesting dynamics (although it would allow people to stall for potentially a very loooong time).
Perhaps we could add a different type of time control so that there is a certain amount of time for the whole game (i.e. x days/months for the entire game rather than x day(s) per move)?