If you want an alternative to gelatin for coating negative-producing plates, just get into wet plate collodion. The chemistry is messy, the results are irregular, but you are dealing with a 100% animal-free product. Silver nitrate, collodion (cotton fibers dissolved in alcohol), ether, cadmium bromide, potassium bromide or iodide, sugar, ferrous sulfate, and cyanide (or hypo). All plant-based or inorganic.