Not possible to do by pole? I mean, I think I have 1 like that and I do it with my 6m Pole.....doesn't clean as well as app + Squeegy, but still, a fall is a fall, and can have disastrous consequences....
I have one or two with a window with a sloped slate roof at a slight incline.....VERY slippery, so if it's not dry (dry is perfectly save) I always use my pole...yes it doesn't clean that good, but it's clean, and I still have both my legs when I'm finished.
Your life or a window....easy choice.