hey holmburgers -
the way the gelatin tintype plate worked is just like you are talking about
the emulsion was painted on a black surface ( blackened tin, black glass )
made the exposure and then bleached it +fixed it.
rockland colloid's process works the same way ...
the clear bits don't bleach at all ( nothing there, so they show the black behind it ) and
the black bits get lighter/bleached ... the mid tones stay around the same.
i am sure if you put some of your azo emulsion on something black you could probably do the something similar ...
without exposing yourself to cyanide