I went straight to the MAC counter and the girl who helped me said she'd done the very same thing to her own brows when she was in beauty school. She said it would take a good month at least to grow out, but in the meantime, there's a fix.
I bought MAC's Lingering brow pencil and Saddle eyeshadow to fix my brows. Close up, it's definitely noticeable:
But from a slight distance, it doesn't look too bad. I can live with it. I'm not sure I can live with my stupidity, but I can live with the MAC eyebrow-hole fix!