Why yes, yes you can!
Thanks to a reminder from Tawnya over at One More Chapter, I can now autograph ebook copies of Future Letters. I did my first two this morning (which were also my first two autographs I've done for someone I didn't know). Authorgraph is a great idea. Only negative - if you want Laura-Glue (who was the inspiration behind the character Skylar) to pawograph your copy for you that can still only be done on the hardcopy version of the book.
It works for all e-readers. So if you'd like an ebook autograph my author page on Authorgraph is here.