The Star Wars autographs website Wattographs is, as they have been doing since Celebration II, offering fans who cannot attend Celebration VI a chance to get things signed by the guests. You can either get a signed print, or you can actually send in an item to Wattographs (it must be sent in by 15 August) and have them get it signed for you!
Ackbar-related guests at Celebration VI include Tim Rose (the man behind the Ackbar mask in Return of the Jedi) and Tom Kane (who is probably most famous as Yoda in The Clone Wars, but has voiced Ackbar in Star Tours and the video game Battlefront). There are two Tim Rose Ackbar prints in addition to the send-in option.
Check out the Wattographs site for more information.