microcord

I can't speak about brevets but as for club rides yes I have seen it. (And in the very same Tokyo!)

For long rides I bring along a lock that I never otherwise use. It has an unnecessarily and wastefully long cable, thin enough to be easily cut but of course more of a deterrent than nothing at all. It was sold wound up in its own little wallet, and when stuck back within this it's rattle-free. I can't contribute anything else to group rides, but ask "Anyone want their bike locked up to mine?", which often gets grateful responses.