I have to say, I much prefer the suggestion to set a threshold for each basic need. It seems sensible to me that during a siege situation, any soldiers on duty (or alerted, or whatever term you want to use) but not engaged and without orders would be doing their best to grab food, water and sleep, even if they aren't particularly in need, since they don't know when they will next have the chance. Then, when they do have to get stuck in, they aren't tired, hungry or thirsty. I think some kind of 'combat time' is too unweildy, having to disrupt the flow of time, further breaking down the game into distinct, dare I say it, mini games. Its also fiddly and its just a more complex routine to go through, having to turn on, or turn off a particular mode every engagement. Or worse, having it snap to it every time urist Mceasily scared engages in fisticuffs with a fluffly wambler.
That and setting needs threshold isn't just something that is useful for the army. I'd have my doctors using it too and the ability to set conditions improves player control. Now, in terms of it being exploitable, its not a major problem, presuming we let dwarves exert their personalities when the threshold is set too high or low. A dwarf wont go without food water and sleep just because you ask him to, neither will they spend every waking moment trying to eat, sleep and drink constantly, even if you ask nicely. Either a fixed number of settings - 80%,60%, 40% etc or a sliding scale that either stops somewhere between 0% and 100% or allow dwarves to break those settings if they are too extreme.