Well, for most people, adding a battery isn’t going to be an option. I tied both of the battery pins to ground, as suggested if you don’t incorporate a battery. That said, if one were dedicated, they should cut the pins off of the RTC chip itself and solder battery leads to them instead. There is almost certainly enough space in Flash to add a check to see if the clock is set or not on boot (currently compiled sketch size: 30,796 bytes of a 32,256 byte maximum).
Sorry if this is a hassle. It really does draw minimal power on low brightness setting, and barely anything when the display is turned off (.5 Watt), so leaving it plugged in seems practical.