It's my understanding that while a capacitor won't work as an energy storage device in an AC circuit (as is charges and discharges with each cycle), it does function, albeit with reactance lag between the input and output, which is typically by 90*
While I've never tried this, but it look as if wiring a capacitor into the lighting circuit (and with this 90* reactance lag), you gain some brightness at idle (as the capacitor is remaining charged -- in either + or - -- after the input voltage reaches 0 on the sine wave), but I don't think it will make a difference at RPM's.
Again, this is just my theory based on my understanding of AC capacitor use. The only time I deal with caps in my professional work, it's DC.
But for a tiny bit more work, you can half/full wave rectify and not have to worry about it and just know that the capacitor WILL act like a battery and provide better output at idle.