I've been to 2 Joecons (Disney in '04 and Minnesota in '05) and it was a great, fun time. They sell lots of exclusive sets and I give credit to Master Collector as they put forth a fantastic effort for these Cons and the GI Joe Club.
Thursday is usually sign-in day if I can remember correctly. You sign in, get your exclusive set and it's done. Friday, you line-up, you can buy some of the more exclusive vehicles, yaddayadda...the doors finally open and usually everyone rushes to Hasbro's area (they have a pretty sizeable set-up) that shows off all the new Joe stuff. Sometimes they show stuff later as well during their Roundtable discussions.
I really doubt Sideshow will have anything there...especially if that is the only prototype Snake-Eyes. I believe that they may give Hasbro some info. (perhaps a sneak peek at a head mold or a list of character) to show off at their roundtable (cameras/video ARE NOT allowed during these) and maybe Hasbro will have a photo of the SE figure in their glass display cases.