Overmolding liquid silicone onto metals is normally easier than thermoplastics. Similar approaches (primers, adhesion promoters, and treatments) can be used to encourage the liquid silicone to bond to a metallic substrate, however these are not typically necessary as liquid silicone tends to bond to metal much more easily than plastic.
Metals also allow for the option to more easily create mechanical silicone bonds. They can be shot peened, sand blasted and surface abraded, these all create more surface area for the silicone to grab onto during the liquid silicone overmolding process. One thing to remain wary of when overmolding liquid silicone onto metal (or any substrate for that matter) is inhibitors, some metals are made with a high concentration of sulfur so that they can be more easily machined, the sulfur acts as a liquid silicone cure inhibitor and can retard or entirely prevent the crosslinking of the liquid silicone matrix. High temperature is also rarely an issue when bonding liquid silicone to metals.