For Mac OS X, what is required for HDCP (video copy-protection) on an external monitor with HDMI?
The answer is that I seem to have encountered a bug in the implementation of dual displays in Mac OS X 10.9.1 Mavericks.

If I am using both the internal display and the external display at the same time, without using "Mirror" mode, then iTunes will not permit playback of the iTunes HDCP-encrypted movie on the external HDMI screen.

However, there is a workaround: if I close the MacBook Pro's lid, disconnecting the internal display, so that the MacBook Pro is only driving the external display, then iTunes will play the HDCP-encrypted movie on the external display over HDMI, in full-screen mode -- although iTunes still displays the same error message saying that it can't play back! 

This is quite inconvenient, because I want to use both displays at once, without Mirror mode, at all times, so, for example, I can have Apple Mail and Microsoft Word up and running on the internal display while I'm watching the iTunes HDCP movie on the external display. 

It looks as if I can further work around this bug, by first starting iTunes, then closing the lid, then starting the movie in iTunes on the external monitor, then raising the lid on the laptop again to activate the internal screen, then continuing to use other applications on the internal screen while the movie continues to play in iTunes on the external screen.