Just suggesting that we be able to clock minecarts with chests. http://www.minecraftwiki.net/wiki/Minecart_with_Chest
I'm not sure this is even possible, since the minecart is a moving entity. And the way locks work, is they lock a certain block (coordinates), not the actual object you lock. Since its a moving entity, I dont think it can be locked, because that would require update the lock every second or so to keep it safe