I've seen that display on my 2004 pre facelift W203 so it's definitely possible.
From memory, I think you need to turn the key to position 3 (the last one before the starter). make sure mileage is displayed in the cluster then press the reset button three times, this will display UB followed by the voltage. Pressing the UP/DOWN arrows on the steering wheel will show other information such as oil level. I think if you turn the key to other positions with this hidden display showing other functions are available, including the famous dyno/invalidate your insurance/kill yourself mode.
Note this is from my failing memory so the exact details may be slightly different.
You can also see the voltage on COMAND, assuming you have it.
Set it to Radio (anything but Nav should work)
Press and hold the Mute + 1 + 3 buttons simultaneously. You will need to hold them for five to ten seconds.
Eventually a scary screen will appear. Select 'Versions' (press right knob) then scroll (turn right knob) down to 'infos' on the next page. Select by pressing the scroll button and you'll see battery voltage, how many satellites COMAND is seeing, your latitude and longitude, your GPS speed and rather usefully, whether you're going forwards or backwards!!
