Remove stems and seeds from the chiles and rinse. Cover with hot water in a small saucepan, simmer gently 5 minutes, then turn off the heat and soak 15 minutes.
Blend the softened chiles with 1 cup soaking water, half the onion and the garlic until smooth. Strain if desired.
Pat the pork dry. Heat oil in a large heavy pot over medium-high heat and brown the pork in batches. Return all pork to the pot.
Add the chile purée, broth, remaining onion half, oregano, bay leaf and salt. Bring to a simmer, partially cover and cook gently 1½ to 2 hours, until the pork pulls apart easily. Skim as needed and add water if the broth reduces too much.
Discard the onion half and bay leaf. Shred or cut the pork into bite-size pieces. Add the drained hominy and simmer uncovered 20 minutes.
Stir in orange zest, taste and adjust salt. Serve with cabbage, radishes, chopped onion, lime and tostadas.