This way you'd see a different set of custom directories for each source directory of a given opened image.
Imagine you open My Pictures and it's a bunch of random stuff, we'll say nature shots, food, and your kitty.
You hit F7 with an image from that folder open. You can send things to Nature, Food, or Kitty.
Later on you want to sort your Nature folder so you open it.
You've got night shots, day shots, the moon, etc.
You hit F7 with images sourced in this folder and you see: Night, Day, Moon... Etc.
It actually did this to me accidentally before but apparently it was a bug of some kind or my mind fabricated it from other contexts.
Also I'd like the option to keep that dialog open on the side so I could quickly sort large numbers of photos just by pressing numbers.