So, I asked your question, sort of, in the Pottery Heads group on Facebook. Here’s the best answer I’ve gotten so far:
Gerald Rowan
Verification and glaze maturation is the product of both time and temperature. A long soak at 9 may mature cone 10 clay and glazes. Problem is how long to soak at 9. The only way to be sure is to test. The test may need to be multiple firing at 9, extending the length of the soak with each test. The kiln will probably be nearly empty of ware (No sense in wasting ware until you get the time right). Put in a few cylinders glazed with your palate of glazes and some test tiles. You can use old fire brick to estimate the mass of a kiln full of ware. Also place a cone pak in the kiln (cone8, 9 and10). If your making your own porcelain clay body and glazes you can add a body flux to the clay and additional fluxes to the glazes to adjust maturation temperature of both.
My question was whether one could vitrify Cone 10 porcelain with a long hold at Cone 9.