I believe that you need a minimum of about 0.020 to 0.040 amps (20 to 40 milliamps) at initial switch on to drive the voltage regulator and energize the rotor sufficiently. Some LEDs are advertised as "12V" LEDs which have an integrated resistor inside them to limit the current, while a normal LED doesn't and requires that an appropriate external resistor be used depending on the voltage it will be subjected to. A typical non-integrated resistor red LED will have a voltaage drop of about 1.7V, a green LED about 2.2V You can assume 12V will be applied. Many small LEDs are rated for up to 20mA or 30mA of current, so you could make it work with appropriately rated discrete LED and say, a 470 ohm, 1/2Watt resistor or an integrated 12V LED as long as it will pass at least 20mA with 12V applied. It is entirely possible that the LED indicator on that unit has a series resistor already, but I didn't see any info to prove it or whether its connected "through" so that you could wire from it to the wire which goes down to the voltage regulator terminal.
I think something like that unit would look very cool inside a /5 headlamp shell!
I would contact the supplier for more info to determine if it would work directly or would need some external additions to ensure that the LED indicator on it worked with our wonderful Bosch charging system.
