Do you want this 20 sec. video clip to play before the ???? What do you have in the iDVD project? A slideshow of stills created in iDVD or a movie/video file?
If it's the slideshow of stills you can add a video clip at the beginning of the slideshow. iDVD 7 not allows video clips to be added to slideshows:
If you want the video clip to play immediately when you put the disk in a DVD player and then fall out to your menu just add it as movie to the menu and then, in the Map mode, drag that clip into the Auto Play bin.
The clip will play and then fade into the main menu.
If those are not what you're trying to do describe in detail exactly what it is.
OT