I knew that such a television series had been proposed in the late 1970s, but was unaware that it was more than a conversation between producers and financers until just today. Apparently some sets had been built, some actors cast, and even test footage filmed.
How much of this exists? Is there so much as a partial episode? Is any of it available?
According to the Wikipedia page
Several minutes of test footage, including a view of a redesigned Engineering Room, costume tests with crew, screen test footage of David Gautreaux as Xon and costume test footage of Persis Khambatta as Ilia, were included in a featurette on the DVD release of the Directors Edition of Star Trek: The Motion Picture.
Also on the same page
Although the series was never filmed and is thus not considered part of the Star Trek canon, several sources, such as the Star Trek Chronology, acknowledge the basic premise of the series, and have Kirk conduct another 5-year mission after the events of Star Trek: The Motion Picture.
So, no full episodes, but some test footage.
With regards to the scripts for Phase II:
Two scripts for the series ("The Child" and "Devil's Due") were rewritten for use in Star Trek: The Next Generation due to a Hollywood writer's strike. "Kitumba" and "The Child" were filmed as episodes of the Star Trek: Phase II fan series.
