I know we say never to do this to ponies, but....
"OXY-10 acne medication is 10% Benzyl Peroxide, it can be really useful for gently bleaching stains out of vinyl.. It is available in several brands, including generics. Just look for products in the acne/skin care section that are labeled as 10% Benzyl Peroxide.
You probably already know that OXY-10 will remove ink stains from vinyl, but did you know it will often remove marker and dye transfer stains as well as other stains deep in the vinyl?
To use, gently wash the affected area with soap and water. Use a cotton swab to apply a tick coat of OXY-10 over the stain you wish to remove. Place your dolly under a desk lamp, or in a window sill so that the stained area is exposed to bright light. If you opt for the windowsill option, please consider wrapping or covering the rest of your dolly with a soft towel, as the sun will bleach her clothes and vinyl over time.
Check your doll every few hours by cleaning away the OXY-10 an applying a fresh coat. If you cover the OXY-10 you have applied with a small piece of plastic wrap, it does not dry out as fast, and you will see results sooner.
Depending on how bad the stain is, it can take several hours to several weeks to completely remove the stain.
It took about 24 hours to remove moderate ink on a dolls hand.It took me one week to remove dark ink from a dolls face.
With the OXY-10 treatment, patience is the key.Your dolly will thank you for it."
http://agplaythings.proboards.com/board/217/miscellaneous-doll-parts