Hola EastBay,
Have you actually done this route MDZ-SCL-USA as a SCL transit? I only ask because I have also heard of reports of the above (being forced with luggage to clear immigrations and customs at SCL because of distrust of MDZ security). I think the OP should be prepared for the worst case scenario unless it can be confirmed by a recent real life traveler who did this without that hassle.
Remember, it is only those SCL-MDZ flights that even make MDZ an "international" airport.