I've never used iDVD. Never even seen it, actually. So I don't know what it can or can't do.
But in DVD Studio Pro this is trivially easy. Just set the end jump of the track you want to loop to the same track. In other words, if it's track 1 you want to loop playback, just set track 1's end jump to track 1. When the DVD gets to the end of the track, it'll jump back to the beginning again, and so on until you mash a button.