on pg 109-110 of Ash Glazes by Robert Tichane, a technique is described to collect rinse water from ash. The soln would contain potassium carbonate, NaCl and trace amounts of potassium phosphate and sulfate.Brian Persha use to draw on his pots using that technique back in he 1970. He had a studio in Red Lodge, Montana.
this may sound stupid but there is a potter, mckenzie smith, in florida who covers an area of a raw pot with wax, draws through the wax into the clay and fills the tiny, thin line with black glaze. these lines are hair thin. firing once to remove the wax leaves the pot clean to accept the clear glaze.
could this work for you? wiggly lines could look like horsehair.
it is a very interesting and precise technique.
If a ware were sprayed with a light coat of this solution, then fired to cone 8, it would have a light varnished effect with a slightly glossy surface.
With that in mind, I propose to modify what oldlady and marcia suggested.
1. Spray the outside of bone dry ware with ash rinse solution, as above.
2. Apply wax over the spray.
3. Draw lines, feather design as desired.
4. Apply mason stain, underglaze, or oxides into the etched areas. Wipe off the extra.
5. Glaze the inside of the ware with a food safe liner.
6. Slow single fire to vetrification.
The resulting ware should have varnished surface, black design from the etching, and a food safe interior.
I have not done this before. It might just work.