For whatever it's worth, the ambrotype effect (scattering from the silver image showing as positive against an absorptive black background) is perfectly operant with gelatin emulsions; I know of no reason you couldn't make ambrotypes with dry gelatin plates (some purists may claim the gelatin emulsion won't look just like the collodon, and they might be right, but developing with collodion chemistry would be a good start).
From what I hear, though, don't trust the Rockland Colloid "tintype" developer; it's bad about as often as it's good, and their (gelatin) tintype emulsion (which would also work for ambrotype) is too expensive to waste that frequently...