I think I figured it out... The Zener does act as a voltage regulator. Hence I guess it is just a matter of matching the LED with the Zener voltage of diode (or vice-versa). For instance, the Mk5 Zener voltage is 5.1volts (MAZ40510M) so I guess it would be safe assuming the LED voltage in more or less 5 volts. Here's the actual circuit...
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