On your idea piper, I have made small peices with a square/rectangular hole that takes a ribbon for use as a bookmark. In that context we could tie it in like a medal, or do a push pin/tie tack back, or a post earring - all those (except the ribbon) are simply different findings for the back. Traditional pin finding could work as well for a kilt pin and extra control.

I have never tried to bond brass to pewter. I have some thin brass that I could try to place in the mold with a repouse style perhpas, but wouldn't get much detail. Brass being copper based casts high temp ~1700 deg, where pewter being largely tin melts in the 400 deg range. Another difference is that pewter is usually spin cast to develop density and about 400 rpm. that would also complicate mold matching. Not that any of the traditional techniques have to be used, but... My usual process is to make a master and from that make a rubber spin cast mold of a single peice. Where there true production runs, I would then make a multi peice mold so 1 spin would output 8 peices for example.

Color combo based on metal is a bit complicated and more expensive...

Another option suggested by Tyger is the notion of a silver base. To silver, gold accents can be added. An option with silver is the precious metal clay, a clay like silver and organic binder that can be shaped 3d and then fired. The result is pure silver but slightly porous. This base can take a gold accent with a very durable binding...

Let's talk more on design bases and maybe a sketch or two. I have to run out now, but will try to draw an idea or 2 tomorrow.

Leif