I'm currently running a test strip of film.
I cut some unprocessed slide film, put it in a solution of sodium sulphide, until it went dark, washed.
I tried tin chloride in acidic environment, aluminium hydroxide, among others to no avail.
Taping a piece of tinfoil to one end (hoping that it'd be close enough for the attraction, assumed I would have to dissolve it to get sufficient contact), and soaking it in a sodium chloride + sodium bicarb solution, then transferring it to a solution of potassium ferricyanide and potassium bromide.. and this seems to have worked completely on the strip.
So if I tape the foil to an unexposed area on the roll with images, that should transfer through that hopefully. Although I wonder if there'll be areas that'll be surrounded by highlights that it wont be able to transfer through.
Last option would be dilute HCl.