Use a diamond wheel on your dremel or a jeweler's saw and cut a slit in his forehead where you want the bib of the cap.
Cut the bib out of thin brass sheet, making it a little long as a tab to fit into the slit you cut.
Glue the bib into the slit; since it won't be cast, super glue is fine.
Add a bit of curve to the bib's outer portion.
Use a thin layer of GS to add thickness and texture to the bib (if needed) .
Sculpt the cap over his head (I don't think you need instruction on this part, yes?).
Note: If you check out some of my recent posts in the Boxer Rebellion WIP, you will see that I did this very thing with the scorcerer's high collar.
This a a bit tougher to explain. You could do it in a couple of ways. First you could find real chains and glue them on. They do make some amazingly tiny chains, but still too big for a mini.
To sculpt it, I'd approach it as follows:
Roll out a cylinder (snake) of GS to the approximate thickness of the mass of chains.
Apply it around the neck and to his chest, sealing the ends together.
Looking at your reference art, shape the whole mass, sealing it to him, paying attention to how the mass lays on him and the way gravity affects it. You may need to remove or add putty to get it right. Take time here and really study the mass from all angles. Clay Shapers are the right tool for this step.
Start cutting in the individual strains of chains using a blade. Pay attention to you reference art and the way they twist and flow. Don't worry about making them look "Chain-like" now, just concentrate of getting the strands cut in and flowing right.
Very lightly, with a touch lighter than a butterfly's fart*, dab at each chain strand to texture its surface, giving it a chain like appearance.
Note that steps 3 through 5 will take a long time and probably won't be done until the putty is starting to set up. This would take me about 30 to 40 minutes; when I was learning to sculpt, it would have taken one to two hours.
* Butterfly's Fart is copyright Gene VanHorne