I don't know anything about jewelry but I have certainly seen an increase in feathered jewelry on TV and locally in the last year or two.
I do know quite a bit about fly tying though. I used to fly fish a lot and tied many thousands of flies myself.
The capes and saddles of chickens are the parts that are so coveted by tyers. Although there is also a demand for others types of feathers, pheasants,peacocks,et al. I will only address chickens here.
In chickens they are looking for long hackles with very little fussyness on the base of the feather. They must be stiff and strong but at the same time thin. They wind these feathers near the front of the hook so that the fly will float on the water (dry flies).
Metz capes and saddles were considered top of the line when I was tying and it was common to pay over a $100 for a premium cape back in the 80ies.The metz capes (sorry I am not even sure if I am spelling this right) and saddles came from a farm that specializes in raising birds for that industry. They have very security facilities and claim no bird leaves the premise alive. They have bred birds for many, many generations for the industry and it would take a great deal of dedication to compete with them.
On the other hand there was also many capes available that were not of such great quality, but also serve the industry well. Many of these were dyed and most used for wet flies which don't have to float. If you were interested in exploiting this market that would be where to start. You must keep in mind though that a cape must follow up the head, many are cut right out near the eyes to keep the very small head feathers available for small flies. This means you must kill the bird in a way that allows you the time and luxury of slowly and carefully skinning it. No head chopping, bleeding or anything that may sully or damage the feathers. Consequently gassing is the method of choice and most people that raise birds for the fly industry have no interest in the meat. Although I don't really see why with a little extra effort both could not be achieved.
I don't know how the capes are treated after skinning but they were course and hard so I expect it was a simple drying process. You would also have to fumigate them in some manner. Getting insects into a fly tying room can be disastrous. Anyway a long winded answer to you inquiry, hope it helps.