I started experimenting with an envelope follower designed by Harry Bissell as described here.
I got it to work but had to change capacitor C3 to get smooth envelopes ( if you follow the muffwiggler thread, which I did after bread boarding the circuit, you’ll find that other people suggest a similar change to the circuit ).
As you can see, the yellow line follows the blue signal’s contour quite well. I think I will make a selection switch so that I can vary the ‘smoothing’ capacitor.
Of course, your mileage may vary depending on the input signal.
B.t.w. if you change the diodes in the rectifiers of the peak detectors into LEDs, you can get a nice shimmering light from them.