[Ulzgoroth]

Why would you need any special rules? We've already got perfectly good rules for a knife attack to the neck and for attacks from various facings. And sort of ok rules for surprise attacks. There's no reason to cross over rules for a garrote, most of which are just implications of the general rules in the first place.

A sneak attack like this would usually be done as an All Out Telegraphic attack, since your victim is going to have severely limited defenses if any and there shouldn't be anyone functional to take advantage of your defenselessness. A neck (or neck arteries, as the case might be) attack is fairly likely to not directly kill the victim, but if you've got Serious Bleeding in effect that has a good chance of finishing the job even if you don't bother to cut them some more.