I don't know of an address or any other method of contacting her. I know some her of communicated via MySpace. But there is an the issue of privacy as well.
Maybe someone that is part of this forum that can contact members of the Airplane, including Grace, could ask that an email account be set up specifically for "fan mail" that they have access to. I would like to at least say hello to Grace and thanks for her music.
Or perhaps if they don't want to deal with email, maybe set up an account for them here. All of the messages can be sent using the PM function, which would serve as a more controlled form of email.