I saw your picture -- wow! I'm glad you weren't in there. Widowmakers are a serious threat and sometimes they come out of the blue. Looking up and around before pitching a hammock (or tent!) is one of the best ways to stay clear.
In the photo you sent, you were very lucky it was a tree/branch of that diameter. I don't think any small cord tied as a ridgeline would stop a crash like that. In fact, I wouldn't recommend it as a safety feature (like a crash bar for a hammock). It sound good, but I don't know if it is sound judgement or an alternative to good site selection.