Wool would have been my first choice too, but since you can't use wool, Polydown is my second choice. What will also emphasize your feathers is to do a tight fill around them. That makes them 'pop' more. Quilt the feather in a heavier thread that maybe contrasts with your fabric. Then do the fill in a thinner matching thread.
The quilt below has none of these suggestions, except for the tight fill ;-). This customer prefers cotton batting, but if I'd used wool or Polydown the feathers would have 'puffed up' a bit more.