I saw a tip on another forum for cleaning alloy wheels with hard baked on brake dust, I must admit I was very skeptical but thought I would give it a go.
Quick wash with car wash and cover with Mr muscle oven cleaner and leave for 30-60 mins depending on how hot a day it is.
Light brushing and rinse off and mint clean wheels, all for £2.50 ish.
Now for you purists that will say it's caustic etc, firstly no more so than alloy wheel cleaner and not as damaging as road salt.
It works and the finish has not been damaged. :D