If you are going to fire
If you are going to fire polish you will need to ensure the piece doesn’t devitrify which will be the tendency after firing the surface against fiber paper. A very light sift of clear powder (as you suggest), an overspray, or a light sand blasting will all do the job.
My preferred way to finish fossil vitra is a light sand blasting and then slump. That gives you a matte finish that shows off the imagery nicely.
Helios Kiln Glass Studio