I've been using the PIP as well in the past couple of months, and I too would like to be able to associate a button with an account.

Warrior - F1
Shaman - F2
Hunter - F3
Warlock - F4
Druid - F5

This way when I hit F1 it will always put the warrior in the main window, F2 will always put the Shaman in main window and so forth. If the Warrior is in the main window and I press F1, nothing will happen.

Thanks for all your continued support on key clone!