Fixed! It's not perfect, but the hole is gone, and you do have to look pretty closely to see what I did to fix it. I decided that mending didn't work (I tried, and I could see the patch). Reverse applique was an interesting idea, and if the quilting were heavier, or more random so that I could be sure the patch would be quilted over, I might have tried that. But I figured my feathers were likely to end up with an empty space right over the hole, no matter how I patched it. And I liked the idea of appliqueing over the borders, but not the effort involved in that
So, I did a tiny bit more unstitching, cut out a section of the border and replaced it. You can see the seams if you look closely, but other than that, I don't think it shows as a patch at all. And even knowing where it was, I had to hunt for the added piece. Of course, now that I've found it, I can't un-see it. But I am happy with the result - although I could wish the patch was in the bottom border, rather than the top.
The finished quilt (minus binding)
[ATTACH=CONFIG]403600[/ATTACH]
The patched section
[ATTACH=CONFIG]403601[/ATTACH]