It's not exactly what you want, but have you tried binding "Move focus to next window" to alt+tab? Then you can use alt-tab-shift to cycle focus backwards, at l

It's not exactly what you want, but have you tried binding "Move focus to next window" to alt+tab? Then you can use alt-tab-shift to cycle focus backwards, at least something. (I always forget where that setting is, it's in Keyboard->Shortcuts->Keyboard).