How can I tell iTunes that a .M4V (movie) is an HD movie?
There is an HD flag you can set in the file's header. You can't set this flag from iTunes.

The best solution on mac is subler.

It was updated to support the 1080p value, so you can set the flag to 1 (720p) and 2 (1080p), the only values recognized by iTunes.

You will find the flag in the other settings tab: