Testing tags for generating shiny dresses by trying stuff that a Google search suggested. Results:

"Latex", "PVC", "Rubber" and "Vinyl" work very well basically always.

"Plastic", "Leather" and "Spandex" also produces very good results. Sometimes even better than the above. Their styles tend to fluctuate though.

"Polyester" works some of the time and generally felt weaker.

While "Metallic" generally worked, it, "Gold" and "Silver" seem to be treated more like colours than shiny materials.

It basically completely failed to recognize "Glass", "Mirror" and "Ice" as being related to the dress.

"Satin" looks nice. Really hard to get it to be shiny but it's nice in general. :D

And I guess "Velvet" just kinda refuses to ever be properly shiny.

"Gleaming Oily" and "Wetlook" help a lot.

"Reflective" helped sometimes but also completely messed things up just as often.

Conclusion: I probably need to test this in a non-realistic model to see which words actually work. It's also probably heavily model dependent in general. "Latex" has worked for me so far as an all-around shiny outfit tag.