Your zip has both static AND dynamic loads... get one wrong, and you've dropped someone on their heads (or butts), usually at a relatively high rate of speed. Add the height element, and you could be sued for negligence. Hate to go there, but in todays litiguous society, unless it was for my own personal use, or you have a structural engineer sign off on all aspects of this thing, you can expect to do the Head-on-Platter routine when something goes fubar... I watched people build a relatively short one (250 feet at a min elevation of 14 ft.) and it collapsed or broke - in different and spectaculay ways - every time they tried to use it. End result was a broken leg, a broken ankle, a f'd up back, a concussion, etc... Put it 30-40 + feet up, between platforms, and have that thing fail...
Or have the brakes fail on your carriage, or abraid the wire rope and have the rope fail, or... or... or...
I hate being a wet blanket, just know that if you do this thing, even for someone else, and something goes wrong, guess who they'll be looking for. A zipline is one thing... the loads of the carriage and brake add huge complexities to the loading factor. Me? I love ziplines and cool projects like this, but I wouldn't touch it with a 10 foot pole.