Physics bridge collapse
It didn't work very well. As soon as I enabled physics on the objects concerned, it started to behave a bit oddly. Two corners of the bridge began to swing downwards. The sim began to run like treacle. And then the whole thing began to break apart, very slowly, with rings passing through each other and shooting off in a rather interesting but utterly unproductive way.
At the end of it I was left with the base bridge board half through the stream bank, with the departing links scattered across my land. Though I don't have much land, so it wasn't hard to track them down and delete them.
I suspect that non-physical objects may be the way to go in future, but I'm determined to get something interesting working on a permanent basis. Earlier on I built a small installation with similar links chaining a scripted physical "creature" to a post, which constantly tried to get away. A similar thing happened there, with breaking chains. Perhaps SL was just concerned about cruelty to scripts.