There's also a "no eye-glasses, no bare asses" clause in Welling's contract.
Now, THIS I agree with. After all, who would want to see Welling's bare ass??
Durance, Mack, even Kruek, sure. But not Welling.
As for him appearing as Superman in the series finale, yes he should. But if he doesn't want to, there is a way for us fans to have our cake and eat it too. Simply use the shot from the 1978 movie where Superman flies out of the Fortress of Solitude just before we cut to Metropolis, but digitally put Welling's head on Reeve's body.