As far as I remember it has not been tried, and some alufoil might actually add a tiny amount of luminance to the center bit by recycled light.

There are some leds however, leds with sharply curved domes like the SSL80 but also the 219A, that when focussed a bit above the die, so the focal plane into the dome, produce a round hotspot without visibly loosing luminance. It is an observation that I did but never did any measurements on it.