Sorry, I don't have any suggestions that you haven't tried already but I wouldn't recommend any harsh chemicals like brake cleaner.
Just thought of one. Maybe rubbing compound used for taking off oxidation from your paint. Try one that's the last step before wax, very fine. It certainly won't hurt it. Finish off with wax to help seal it. I use one all the time for boot scuffs, exhaust stains etc. and it works great.
Please let us know and good luck!