You don't get an author to sign an e-book -- rather, you get them to sign something scarce.
For instance, an enterprising company could create something specifically for signings -- an author-specific website for uploading pictures (or videos, etc.) of you and the author; a smart-phone app that lets authors directly download book-tour-specific scarce items to your phone (website badges, scans of signed 8x10 photos, ballot entries for giveaways, etc.); a Foursquare-enabled "check-in" that also allows authors to authenticate it; a physical coupon that the author can sign that fans can scan/fax to the publisher or to a website to be entered into a drawing; etc.
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