Depending on the shape/size of the slots on the wheel, you may have to try to rig up some tools with long handles. A car I previously owned had wheels with difficult-to-get-at insides. I used to use a long handled brush designed for cleaning home air conditioning units. I shopped around for the one with the softest bristles I could find and I still used to hold a MF over it with a rubber band or masking tape out of fear the bristles would be too coarse. With some APC sprayed on the insides, I agitated the insides of the wheels with this brush through the slots. It usually did the trick but I was on top of the wheels religiously.
Jacking up the car also allows more room to work with. I never did this myself because I used to clean so frequently, but for client vehicles it may be an option.
Also, if you're willing to twist and contort yourself underneath the vehicle, ( I still do this, car not lifted) you may be able to access some of the insides of the wheels with the tradition types of wheel brushes; the Meguiars cone shaped wheel brush, for example. You might also be able to reach in there and clay once all the heavy gunk is cleaned off.
It can be an uncomfortable and dirty job. Charge accordingly.
