I think the dress will work great with your style Una and definitely worth a try. I haven't read/heard anything about exposed zippers being on their way out.
As for your rayon question, I honestly wish there was more rayon around, as opposed to polyester and other man-made fabrics! While it is not cotton or silk, it is still a natural fiber in a loose sense of the word and so much more pleasant than any of the non-natural alternatives. For me, it is flimsy summer wear in the same amount silk is flimsy summer wear.
ETS: I believe that it is cheaper than silk and cotton, hence it's relative abundance, or overabundance of polyester. Rayon/modal/tencel etc. are all pretty much the same, and tend to be really soft.