I just rehabbed a set of GTA front wheels. I did not look too hard for a solvent that could removed solidified brake dust. I was able to remove 99% with a variety of screwdrivers and rags. Wrap the tip carefully and rub the black spots until the tip is black, then move to a new spot on the rag. The only part of the wheel that I had real trouble getting the black off was in the tiny holes near the center of the wheel. Masking off the silver lip was a PIA. I cut short pieces of masking tape, worked them onto the wheel from the painted center out with the tip of a key, then went back and trimmed along the paint line with an exacto knife.
The other option I considered was powder coating the entire wheel or just the center portion, but decided to try this first to see how well a careful spray can paint job will hold up.