Balloon rubber is basically just latex.. which can be found in a sprayable form. You may need some sort of industrial spray gun to apply it, though.
I found this while searching around a bit... might be possible to tint it with some sort of pigment to get a nice iridescent white color. I wonder how durable the coating would be on top of Spandex. :confused
I'm starting to suspect that they may have done Quorra's leggings in a similar fashion.
Liquid latex is pretty easy to get and apply by hand to lycra

I have tested this a few times and there are a few things to think about:
To save time and fiddliness pattern the pieces first then cut your base lycra into panels- not the pattern shape but small enough to be able to handle. You can then cut the pieces out of the best spots and the latex will prevent the edges from curling
Do not soak the lycra and then drape over a form. The latex will start to cure on your hands and you will wind up with latex equivalent of pilling- tiny lumps.
You can thin liquid latex with water to the desired consistency.
You can drizzle and paint the latex on to a vertical or horizontal mold. If you mix the latex with water you can start with horizontal and turn vertical to let the excess drip off.
You can colour liquid latex with acrylic paint. Even metallic acrylic pant. It's easier to pre-colour the latex than paint after. You will need less than you think so mix then paint some to cure to test the colour.
Latex sticks really really well to lycra, and it winds up being much stronger than latex alone.
If you need to glue anything to the latex Shoe Goo is amazing. I just repaired a torn set of Asari tentacles with this and it's worked really well. No discolouration and it flexes well.
I have just decided to make this costume too after much internal debate

But given I am having violent reactions to resin I think this is a more sensible costume
I am though using my stash as I have silver and white satin finish PVC already. That and I have just got the smell of balloon out of my workroom
I will probably go reflective tape for this as well. Looking at the BTS footage the EL is pale salmon coloured but the Siren costumes don't. They also seem to be on some roughly cut base:
IMG_4048 | Flickr - Photo Sharing!
Funny thing is when I saw tron at Imax I really couldn't tell the lenses had hexagonal pupils! Saw that they were larger and there was a distinct ring around the iris. So I may wind up getting lenses that give that feel as opposed to proper ones.
Also I'm a tad up the proverbial creek. I have red hair. Admittedly it's more orange than primary red... I think I'll just use a hair spray to go lighter.