Celia, Yes, use your porcelain powdered clay, mix with water, allow to soak for a while and use a blender or immersion blender to incorporate. Sieving is a good idea. Thin with sodium silicate or not depending on the look you want to achieve. Be certain to match the cone requirement of both clays. As mentioned, the glaze colors are brighter over the porcelain slip than over stoneware.
Yesterday I sprayed a bunch of cookie cutter ornaments (white clay, bisque fired) using Krylon Glitter Shimmer opulent opal spray paint. I'm really pleased with the result. This spray paint must be used outside because of the fumes. Not wanting to get glitter paint all over my wrought iron table, I made a hanger from wire, inserted hanger into the top of a large cardboard box and sprayed each ornament suspended from the wire hanger, trapped the fumes inside the box. After painting about 2 dozen ornaments this way, I just laid the rest flat inside the box and sprayed away, spraying two coats of paint. Wow! They are lovely. I did wash the bisque dust off the ornaments before spray painting them.
Oh yes, we use them every day. Most go into the dishwasher, microwave and oven on a regular basis. And I'm happy to say that my son and daughters use them on a regular basis too. My youngest grandson calls his heart shaped bowl "Nana's Love Bowl."