Is this devices rack currently selected?
Is it possible to tell a m4l device whether the rack its nested in is selected by the blue hand or not?
Not sure about blue hand, but if you just want the currently selected rack, you would just detect any selected device, then determine whether it is a rack (has chains), then determine whether the rack is a parent of the current device (you could keep getting the parent of the device, until you hit "track"). A bit tricky, but not impossible.