It's possible in Superliner coach. The thought did cross my mind on California cars, where some are open bin and some are airline overhead style with doors. However, very few people I saw ever used them, although I'm thinking there were a lot more people with luggage on the San Joaquin.If there are no valuables why would you need to lock them to the luggage rack? There's not an easy or obvious way to lock most luggage down and even if you could it would interfere with other passengers trying to add and remove their own luggage along the way. In all my travels the luggage has still been there when I was ready to disembark and the few times it was messed with it seemed to be children doing it. A few zip ties should be enough to keep curious hands at bay.
But there are those metal separators on Superliner coach cars that could theoretically take a loop lock.