I know it can be done, but I don't know how. I have a set of fakes made on retainers I got years ago from the outfit inthe U.S. that no longer exists.
If you could figure out what sort of plastic the retainer is made from, maybe on a orthodontic supply website, then you could figure out which epoxy or adhesive would bond metal to that type of plastic. There used to be a website that gave information on what adhesive to use for the materials you were trying to bond.
NapaCaster