There is nothing official specifically for Cast Members' wheelchairs that I am aware of.... Back in 1997, I was in the College Program....when I checked into Vista Way, the counselors just told me to keep the chair clean, and not to add anything to it that would get guests' attention. Same general rule of thumb as the personal grooming and appearance.
Two years ago, I needed to get a new wheelchair to meet my needs. We designed the new chair with my WDW dreams in mind. First thing was to make sure the chair would clear the arch over the "It's a Small World" boarding area. When I started designing the seat, I took the basic rules of the Disney Look into consideration. I am sure that they would allow the positioning harness and headrest if I asked, but they would detract from the look of the costume. I want to be the best cast member I can be, and not ruin the magic for the guests. I have not talked to Disney to see what they will accept as "reasonable accomodations" for the costume. Rather, I just chose supports that can detach or fold out of the guests' sight. I really want to make Mickey proud. In addition, I chose a paint color that would not stand out in the parks. I wanted Gator Orange, but decided to go with "Midnight Blue" to look more professional.
