Bus voltage measurement in a compact way
As I said in a comment to the other answer - a low voltage zener like 3.3 or 3.6V would start to conduct much before it's nominal zener voltage. It may conduct small current like 100-200uA at something about 2V and this way it will seriously distort your divider's output. These zeners need current of at least 30-40mA to get their voltage near their tollerance range.

A low power LED has better characteristics than a low voltage zenner, but I think it will also add some non-linearity to your resistor divider.

Check your ADC's specifications. Check what is the highest input impedance you can afford for your sampling speed. In such cases I prefer to raise the impedance of the divider so its current to be about 200-400uA and place no zener. This 400uA current on bus overvoltage can be carried out by MCU's ESD diode (not recommended!) or by an external small signal schottky diode (for example BAT46W) to Vcc's bulk electrolyte capacitor. This diode will have a reverse current of less than 1uA at 25C temp. and less than 5uA at 60C temp. so it wouldn't mess the divider voltage much. You could also find a diode with lower leakage current.

Also a capacitor of at least 20pF would be good to have near the pin.