Pat, You have shown us the pic sometime ago.
So hidden behind the arch brickwork is a normal window. I guess they dont slide from within, otherwise you wouldnt be asking the question. You need to go up between the glass about 12-18" top left and right corners hidden from view, The gap is only 8" so the brush needs to address the glass at 90 degrees on some sort of goose neck on top of a 30ft pole, but to get there the goose neck/pole needs to vertical and the brush still has to be at the correct angle to clean the glass.
If this is the case, Could you cut/remove the broom handle holder from the brush to leave the brush a flat back, then bolt through the front of the brush through the stock and then through the goose neck with a wing nut and bolt. his would allow the brush to be torqued and swivel left and right to go top left and right of the hidden glass.