At the kennel (dog and cat shelter) I used to work at we used sharpie markers all the time to mark cat cage #s on stainless steel cage name plates and used the same pen to mark other items arond the facility.
We used a little chlorox bleach to remove the marks so we can change the numbers as new cats came in and old cats left. Bleach (even if diluted with water) takes off sharpie pen marks quickly and easily but I don't know if it would be a great cleaning substance for piano keys at all as it may be too corosive.
Bleach is a harsh substance when at full strength (and I had many holes in my work shirts from cleaning cat kennels to attest to it as well as bleach spots on my clothes). Bleach can also burn your skin and is not healthy to breath in as it is very irritating and caustic. I lost my ability to smell bleach after working in the cat room for a period of time (it took about a year before I could again smell bleach). I also got bronchitis frequently while working that job form breathing in all the kennel disinfectants.
Ask your tech or any of the qualified techs here for advice is the safest way to be sure as what might be best for getting permanent marker marks off your keys.
Just my two cents. Don't know if it will be of help for you in regards to keys; but the stuff works in my house for our kitchen table and counter tops when we use it carefully but then again I'd be very hesitant to use it on fine furniture at full strength. jmo
WARNING: I recall one should never mix bleach with ammonia (which windex has in it) as I recall there is a chemical reaction that can happen between these two substances that will occur if these two ingredients that can be harmful or possibly deadly. I think the reaction gives off HCL gas but I cannot remember the exact reaction that occurs as it has been forever
since I had taken my chemistry classes.