Bear in mind that most metals which are good for making a front surface mirror will be affected by the processing of the emulsion. Platinum or Palladium are (perhaps) exceptions, but not cheap. Aluminium, nickel or other shiny, broadband reflective metals will all be etched and/or interfere with the photographic reactions. Gold is yellow.

If you have a set of silver fringes suspended in gelatin with the right spacing, they will automatically reflect light back at the right wavelength. The effect will be strengthened by reflection off the back of the emulsion. The refractive index mismatch between gelatin and air will do this on it's own, but silvering the back will make the effect stronger still.

These days, I suspect the best method would be to vacuum coat the rear of the emulsion with Aluminium after processing.