France Scully Osterman waxes her salt prints with beeswax and oil of lavender.

Stan Klimek mentions waxing Pt/Pd prints with Gamblin Cold Wax Medium. I haven't waxed any pt/pd but I have tried it on argyrotypes and it looks beautiful. There's a small section on waxing by Klimek in the last appendix in Arentz's book.