even making it so at each page turn, the clock display is refreshed, would make a large inpact on battery life? that doesn't seem a bad idea to implement a clock
(in my opinion)
maybe having it so if the page is not turned for 5 minutes, simply refresh the page once to update the clock to show something like '--:--', to symbolize that the time displayed is out of sync and won't be renewed until you turn the page.
the kindle is keeping track of the time already (isn't it?), so i would imagine that 'all' that needs to be done is redraw the clock, no extra processing/waking up needs to be done, at least for keeping track of the time, right?
(i agree having a clock on screen continuously updated would be overkill, that is why i think having it per page is not a bad idea, even so, i wouldn't think the seconds parts are very important to someone, so, already there, you are only updating once a minute.)