I don't know how likely this is ever going to be and like others I recall a similar topic being posted a while back.
It would be interesting to one day put it to the test, i.e. for SM to find out how much it would cost to book the likes of Harrison Ford, determine how much an autograph will cost and how many tickets they would need to sell to make it worth their while (or indeed how many HF would be prepared to sign in a day) and then do a pre-book system with him only being booked if the tickets are all sold by a given date.
I have at least 3 items that I would like HF to sign and would imagine people coming from all over the world for the opportunity (I know I would go to the States if it could be recommended to see him at an event).