That's just it, I *did* do it; it passed at MCO, but was told I couldn't take it at STL (well, actually, the guy at STL took pity on me and let me take it, but warned me rather sternly that he wasn't supposed to, and that I needed to check the bungee next time.)
I don't think that the length of the cord made a difference, as I had taken it off and put in the mesh seat pocket for the pass through the scanner; he didn't take it out of the pocket, so I don't think he could really tell how long it was. I had purposely gotten a cord that had plastic hooks instead of metal, as I suspected that the heavy metal hooks might have set off the scanner. This was the first time I tried to use the bungee method; normally we use a backpack satchel; but we were gate-checking a smaller seat this time. It was only a weekend trip, and we otherwise had only a carryon; we wanted to avoid the carousels if possible.
It seems to me that if you could tie someone up with a bungee cord, you could just as easily tie someone up with the belt from your pants, or with long shoelaces, in fact, those would work better, as a bungee would stretch. Using it as a strike weapon to hit someone would require being able to swing it round first, and I don't think there is room to accomplish that effectively on most aircraft; in any case, the plastic hooks are not heavy like the metal sort.
The folks at MCO didn't bat an eye at it, but then, they also let me walk through with a rather large stainless-steel keychain in my shirt pocket that wasn't noticed either. (DS had just bought it right before we left the last park, and asked me to hold it; I forgot that it was in my pocket instead of my bag.) The experience lead me to wonder whether the inspector at STL was just being overzealous, or if bungees were normally not being allowed.