A number of years ago, some hang glider pilots in the Owens Valley were
using golf-ball sized or tennis-ball sized "mirrored disco balls" attached
to the kingpost. You'd get a bright flash occasionally, and it tended to
draw the eye. I think it did help some with visibility.
Nowadays, the kingposts are mostly gone, and I imagine so are the disco
balls.
And I'm still not much help, because I don't know where the people that had
them, got them.

Tim Ward

I remember those little disco balls (abour the size of a tennis ball)
and I remember that at more than 300 meters or so the flash given off
was no longer discernable, probably because of the very small faceted
surfaces on them.

Mike Z