How about...
When a movie is imported, MC could autmoatically lookup YouTube for the trailer and insert the path into a "Trailer" field.
If MC cannot find the correct trailer, Right Click > Search youtube for trailer (ala cover art), which then saves the url to the Trailer field (rather than download the whole thing). Add the Trailer field to the Info panel (edit) as an option in the Back, Play etc section.
Of course, you could enter the data yourself, but with the right parameters I think the auto option would be great most of the time...