Silver Iodide 'appears' to be insensitive to light due to the fact that it is almost non-developable. So, there may be a great amount of latent image, but you need a very unique developer to reveal that image.

Any halide except iodide or fluoride will probably work, but the barium salts would lead to some severe problems with your effluent. Well, gee, that leaves only chloride and bromide, I guess.

Aluminum halides are not good to use, due to the amphoteric nature of aluminum and its potential to react with gelatin or albumin. Other salts with multiple valence such as iron could work but often interfere with silver halide imaging.

Well, we are back to the alkali metal salts such as sodium and potassium. Now you might want to try ammonium salts, but the ammonia is a silver halide solvent and may mess up the speed of the salt image. It is pH dependant, so if you control the pH the use of ammonium salts should not be a problem.