<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Efrem:
I don't think they can stop someone with no checked bags from buying a ticket to Santiago and getting off in Miami (or the equivalent on any of the other flights like this), but it's an expensive way to get there.</font>
No one is supposed to be leaving the plane at the domestic stop, so I'm sure they would notice that. However, they might not care, and let you off anyway.
Same for the reverse but even more difficult to accomplish. If you bought a Qantas ticket SYD-LAX-JFK, you probably would not be allowed to board at LAX for the flight to JFK.