I don't see how legalities can stop them from showing Clark as Superman. After all, we've already seen the \S/ shield many times on the show; just put it on a slightly altered version of the suit if need be. And if Welling won't wear the costume, set the ending of the series ten years into the future and have a different actor play the role. In any case, seeing Superman in the final scene of Smallville
is a must.