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Get hold of the Association of Challenge Course Technicians for more info
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They're going to say use twisted steel cable, I believe they recommend minimum 14,000 lb. avg. break strength for the zip cable.
I'll assume you're doing occasional zips not permanent or semi-permanent (multi-day) installations.
A static is excellent but a little stretch is good, for example I wouldn't use 10mm HTP but 11mm KMIII would be excellent (a little more stretch). I've used 16-strand 1/2" arb line, works great, very strong for the use. It does stretch a lot so I usually have to re-tension several times as the zip is being used.
This thread is a good discussion of catenary angles and how they effect load at the anchor points:
Tyrolean Traverse discussion
Here's a
catenary load calculator
A good rule of thumb is a loaded zip shouldn't create an angle less than 15 degrees.
Obviously you want very solid anchors both sides but you already know how to assess a tree and determine that it can take the required load. The catanary force calculator helps get you in the ballpark of what kind of loads will be generated.
-AJ
