Fusible backed patches would be the way I would go for a down jacket. Mashing down the feathers would take away their ability to do their job and that would make a cold spot.
Jo
I have not done a jacket but have done sleeping bags and I just hooped the stabilizer and baste stitched the bag to stablizer Came out great
MASH down the design area as flat as you can get it. I dont know how yours is sewn, but if it is squares or sewn in channels, just stretch some water soluable over it and PIN the dickens out of it so it is as flat as you can get it without sewing pins.
yyyaa4. This lady has one in projects. I'll bet she can help
I have not done it, but I will reap the seam close to the area where you want to put the design on and then sew it back close. Another option is using the fusible backing used to make patches.