Back to pit fired slab vases. A silicon sealer would work for a while but probably not forever. There would probably be a need to re-coat it just as one reseals wood decks.
I prefer to think of the pit fired process as non-functional and work on beautiful surfaces that are more interesting than functional.
I have been TOLD that oil lamps need a sealer inside, since the fuel will find its way through the most minute cracks in the glaze. several supply houses sell such a product. Will it work on unvitrified ware? Who knows. . .
Thompson Water Seal works on unvitrified clay.