If you have access to a process that makes a nice smooth surface, go try it. I would suggest that you first try a glass mirror to get a reference as to how the glass system works. Metals tend to suffer from thermal problems more than glass (note the preference for Pyrex over plate glass) and using monel or an even lower expansion metal is probably better. If you do get a nice finish without having to vacuum plate a surface on, and are able to do it easily, then everybody will like to hear of it.