I am not sure about piecing as I think it might be a bit too thick for the seams to be able to lay flat. That said I think for your top thread it would be fine.
I believe some use the rayon thread for embelishments (sp) on their quilts and some for using fancier stitching for stitch in the ditch where they don't necessarily stay in the ditch but in line with the ditch. Personally I only use rayon for my embroidery and my King Tut for all my quilting needs including piecing, embellishing, quilting, etc. Some people love using their polyester also. It's all a personnal preference as to what they like and use.
Many, myself included use rayon, silk, cotton, ribbon, yarn, etc., in our crazy quilts which works beautifully (all of the different mediums of thread). It really makes for a fabulous crazy quilt.
You might just want to make a practice sandwich and try what you're thinking of and see if it works for you. That's really all that matters - if you like it, it's fine!