all german gliders that I know about have a weak-link requirement in their operators handbook. I've always assumed US pilots are required to comply with the operators handbook and the FAA standards.

You can also check the TCDS - online if you like. There will likely be something there regarding weak link strength. For example the ASW-27 FAA TCDS specifies:

"Weak Link
Ultimate strength
For winch tow
1455 lbs. (660 daN)
For aero tow
1445 lbs. (660 daN)"

I've also found bulletins from Cessna giving a maximum breaking strength for the rope or weak link when one of their aircraft is used for towing. Most models specify 1200 lbs. maximum. I only have that in paper form from back in the late 60's/early 70's so maybe that has been superseded in the years since. As I recall it was nothing to do with the hook but rather with the structure the hook was attached to. Unfortunately Cessna's current website is kind of a nightmare and I can't find anything relevant on it.

At my club we're using hollow braid polypropylene and the loop that the ring goes through runs the rope through vinyl tubing so the ring doesn't directly rub on the rope. Once the loop is made and the free end tucked inside the rope to secure it we wrap the loop end in rubber tape topped with electrical tape. I'm not sure how much of a benefit the tubing part is because I've never compared wear rate with and without it. We have a soft grass runway so the ropes have an easy life but I still retire them after one year maximum service time. The tow plane end has a Tost ring and the glider end has a Schweizer ring that the various weak links (made in a similar fashion as the tow rope except that one end is a simple loop of rope) is attached to. We no longer have any Schweizer hook equipped gliders flying at our field but I keep a selection on hand in case a visitor shows up with one.