Here's what I've decided to do for my border.
I tried another quilter's idea of making a border using strips of the cat fabric similar to a piano key border, but with the tops pointed like the cat tails are done in the center of the quilt. But too many tails were indistinguishable from a picket fence border, and, at least in EQ, your eye didn't pick up the visual cue that made you think... oh, those are tails!
I tried coloring it so there was a background fabric, and every other tail was a different calico. This time it looked like my inner pinwheel and cat blocks were enclosed in a cage.
Then I tried putting one cat on the border, and having his tail curl up to be the first strip in the picket fence, but that didn't work either. I gave up on the picket fence - tail repeat idea, and just put a kitty in each corner, then added the yarn balls to partially fill the empty space. I may try to do some small bit of machine applique to make a 'tail' of yarn looping from the ball on the two longer sides of the border. We'll see!