The only level of hope for the Mii would be if they were all restricted to one fighting style and the game was somehow able to tap into the Wii's memory bank and integrate it into it's own in order to construct the look of how you made your Mii on the consol. Though like i said the disc for SSBB would have to be able to customize millions of different looks from the Mii creator and input that into the game. Maybe it's just me or maybe i'm overlooking a huge fact but it just seems like a a feature that will not be included in this game.
I think you're missing out on a feature of the mii's. The mii's are designed so that games can easily and quickly reference them. It would be no hassle, and require no actual space Wiiplay, Wiisports, and Warioware all encorporate the entire Wii, and many other games quickly and easily call on the heads for save games or character profiles. The disc wouldn't have to do anything except for "Call Mii screen" "input X = Mii selection" and "Call input X"