Are you trying to clean the back of the spokes or the inner wheel band behind the spokes?

I have two bricks with Argo wheels and I have modified two brushes, one to allow me to get into the small triangular holes, and the other to clean the larger slotted holes.

I bought brushes that have a rounded head made of wire and squashed them flat with pliers, giving me a long, narrow, full brush.

You might take a toilet brush, squash it flat and hook it a bit in order to work behind the spokes. I use one like this to clean my snowblower when I am done for that session.

Each season I wash our wheels, wipe them down with paint thinner to get the greasy and tarry stuff off, and then give them a good coat of wax front and back. I even wax inside the recessed portions of the spokes. I cannot buff all the wax, but I wipe off all I can get at with a rag wrapped around a stick. They are not smooth, but I am getting years of gunk out and dirt does not stick in there as well as it did.
--
96 855R, 95 855,854, 90 744 Ti - 340,000 Volvo miles put on 7 bricks