My guess would be that they would include a current limiting resistor, or resistors, in series with the LEDs to specify maximum current allowed so the LEDs would not burn out if PWM accidentally got locked at 100% on time. If so, then varying the voltage would in effect vary the current through the circuit as I = (E - LEDvdrop) / R... I would guess a buffering cap couldn't hurt. I shouldn't have called it an RC filter as it really isn't since the LED would be in series with the resistor. At least that would be my take on it. You have to allow for situations where the LEDs might be burnt out and engineer for it in a consumer device, or the first time a processor locks up with the PWM full on it's bye-bye LEDs.