You can change the play movie button from text to a framed movie loop in just two steps.
First click the text of your movie button in the iDVD menu.
Then go to the Buttons menu and choose one of the last four options in the dropdown menu. Click any of the preview images in the panel to make your selection.
The movie will be updated instantly with the choice you have made in the buttons menu. You can click through the choices in the menu until you find the style that you want for the menu.
The DVD menu's appearance with a rectangle frame for the "Play movie" button: