Another vote for a Prusik in the Z-Drag scenario for any type of haul line. With the right size and type of lines, it would likely take somewhere between 1000 to 2000 lbs of force on the Prusik for it to slide or fail. The ascender will start tearing into the rope at some point and could cause a catastrophic failure.
As with anything, do your research, get the gear you think you need, and then go learn to use it to see what happens when you put a few hundred lbs of force on a rope and whatever device you chose. Much better to get the experience on dry land than trying to learn what does and doesn't work at the river's edge.
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