Tom, You asked several questions in your msg. First, regarding lightweight molding. There are several molding made out of styrofoam, which is obviously lightweight. These molding typically are already finished to look like wood. The finishes varying from very light in color to very dark. That's the good news. The bad news is that, being styrofoam, they are not very sturdy. Secondly, the types or shapes of moldings available in pre-finished styrofoam are very plain and simple. ie: you will not find any complex crown molding type finishes.
Re: Can plate rails be appealing at height indicated? When we are faced with design appearances, we go to the local grocery/drug store and buy several issues of home decorating type magazines.
Lastly, re: hanging plates from a wood molding. See #1. If you did use the styrofoam molding and wanted to hang the plates from them, you would need some long screws/nails to penetrate the styrofoam and into something more solid for support.