I tried all sorts of chemical oxidizers, and they all ruined the dial.
I just missed to try very diluted bleach and very diluted H2O2, but I doubt they are safe even when diluted.
I am afraid that only physical means (i.e. prolonged exposure to light) may be safe.
But, as you said, this could take ages (perhaps a little lesser with UV light?).
Oh well, I think I'd better go and place my dial right under this bright sun...