How funny that this should come up now. I've just been thinking about exactly the same thing - and not got anywhere useful.
The engineering problem is that it needs to be something that will only change angle when you want it to, not because you bump it accidentally against a sill or something.
And, as GSW says, it should be accurately adjustable without having to lower the pole every time.
I guess that 4 or 5 positions should be enough as there is a degree of flexibility in the bristles of most brushes and you can adjust the angle by moving yourself fowards and backwards.
I'd prefer something that could be adjusted by pressing against the frame or even a wall rather than the glass.
It's an interesting problem. I hope somebody solves it.