Just got the July/AUG 2104 issue of CatSailor and read all the inputs on tethering. It seems that no one is fully happy with any of the proposed or previously tried jackline and tether methods.

If we look to kitesurfing we might find a couple of good devices. This sport was known for how dangerous it could be until they figured out the combination of automatic depower and a tether that is easily released under load if all hell breaks loose.
1) They don't have an ability to cleat in so power is automatically reduced by ~90% anytime you let go of the control bar. Unlike a mainsheet the control bar force required to hold a kite at maximum power is only about 10-15 pounds of pull. See: https://www.youtube.com/watch?v=xY7lsKTTiE0#t=260 for an example of letting go to reduce power momentarily.
2) If you need to force a stall of the kite (100% depower) you use the quick release #1 which stalls the kite sending it to the water lying on its back. At this point you are still tethered to the kite. See: https://www.youtube.com/watch?v=xY7lsKTTiE0#t=36
3) If all hell breaks loose and you need to cut the kite away, all you need to do is push away on the bypass release on your safety leash (tether). See: https://www.youtube.com/watch?v=xY7lsKTTiE0#t=189

Note: Both safety releases disconnect with only about a 5 lb force when under a 200+ pound load.

Plus there is a retractable leash (like a dog's leash) that is built for salt water. This is used by some to tether their kiteboard to the rider when riding in open water. See: http://www.oceanustech.com/theeel.htm

Note: I am not endorsing either product or brand for beachcats. The videos are just representative examples to give ideas.