I don't think a test sample would work to determine if it is too weak. Isn't the issue with Rayon is that it can't hold up over time?? I don't claim to be any expert. I only used Rayon once (before I knew it wasn't recommended for quilting) and it was a charity quilt. The quilting went fine but I have no idea if it held up.
There are so many gorgeous polyester threads on the market that offer a shimmery sheen. Some are indeed embroidery threads. Isocord is one of my favorites. It has gorgeous sheen. Others are Glide thread by Filtec, Highlights by Superior, Rainbows varigated by Superior. These are only the ones I know of from experience but I am sure there are plenty of others. Glide color Khaki (which is really a lovely rich golden tan) would be stunning on your quilt, which is very pretty BTW.