You could probably sputter coat the polymer, but not the same as metal on metal electro-plating. Even if you did it, it would be cosmetic change only. If you're gonna get a steel framed one, might as well get a Para Ordinance double stack .45....Glocks are polymer too, but you can get aluminum and stainless frames for them. I'm hoping someone offers the same for an XD.
And I don't think you can plate polymer. In fact, i'm sure you cant. BUT i've been wrong before. I hope I am here.