Oh and to Answer your question. I don't think it's ever been done, although I believe it's possible: has a major party candidate ever run for POTUS without appearing on all the states ballots?
Yes. There are cases since 1860 and 1864. Sort of. The parties appeared but with different candidates.
In 1912 Taft was the GOP nominee. But in South Dakota TR had the GOP ballot line and Taft did not appear. In Cali TR had the GOP and Progressive Lines, Taft only got write in votes. TR conversely didn't appear on the Oklahoma ballot.
In 1948 Strom Thrumond had the official democrat line in Alabama. Harry Truman wasn't on the Alabama ballot. Thrumond also had the dem line in Mississippi but Truman appeared as a "National Democrat".
In 1964 also in Alabama LBJ did not appear on the ballot. Only Goldwater and a slate of "unpledged" democrat electors.
Of course I realize if any state won’t allow Obama’s name on the ballot the rats could likley run a slate of unpledged electors (technically the electors are whom you vote for) and if they won they would be free to vote for Obama.