Yes I think its possible, maybe a pack of aa's, or aaa's could be made to fit.
You could use a passive infra red motion sensor to detect presence, you can get them on ebay cheaply.
Ultra high brightness led's can be driven at a low current and still be visible.
If you want to show time remaining for parking, maybe when noone is detected you could just show one led, green for still more than 5 mins left, orange for less than 5 mins and red for out, so an attendant can see from a distance if someone has overstayed.