I read a very different account on that issue.
, Jonathan Harris was asked to cameo, but as the "Nu" Smith's "commander" who double crosses him (the hologram communication). Harris stated he was not interested unless he appeared AS Smith. Mumy, in an act of friendship, decided to pass on his offer as well.
Again, that's just what I read and I'm sure Christopher will gun down the anecdote with unabandoned glee.