From a source online:
"If you have fingernail polish try acetate. Blot the chemical onto the nail polish spot using a Q tip trying not to touch the leather with the acetate and then blot with a towel and repeat, keep on doing this until stain is gone (will take a long time)
From a carpet cleaner of 14 years"
Now, from what I've been reading, is that any sort of household chemical that will remoev the polish will also remove the pigment from the leather. So, you are kind of between a rock and a hard place here. As the quote in the begining of this post says, use a q tip and only touch the nail polish, not the seat.
I know this will sound stupid, but maybe try taping off the spot as best you can to limit any potential damage. I'm wondering if there is something you could put on the seat to prevent the chemical from coming into contact with.