I would just turn the brush through 90 degrees on its side feed it throu the gap in the rail and then turn it horizontal again, assuming its a floor brush type. The gap looks wide enough. Allow a bit more time and price. Theres a knack to it, you can do it if you really want to.
On some narrower ones I just do the bits I can ott and ask then to do the bottom 2ft.
ps Jeff I am locked out of Academy, did you get email 2 days ago ?