Thanks!
With the help of the lolking model viewer and some tracing I was able to make a mockup animation of the ultimate for Avatar Lux! Since I'm not very experienced with 2d animation, I'll try to describe what it's supposed to show: When Lux ults, her wings "detach" from her, and start growing and flaring out. The two separate pieces of each wing drift slightly further apart, with the lower one nearing the ground. As Lux approaches completion of her cast, the wings start to glow brightly, reaching full brightness at the same time the beam fires. They then quickly animate back to the normal position and brightness over a few frames, while leaving behind a glowing afterimage of the position they were in at peak brightness that dissipates after a short time.