[Right. I am determined. This is not about RPGs. This is about a different kind of game altogether.]
I happened to remember that Maurice Dodd, who wrote The Perishers for most of its life, wrote in an omnibus that he'd tried to invent a quite absurd event, and come up with rhubarb thrashing—then, the world being what it is, he'd found (too late) that it already existed. (Moreover, apparently a band called Stackridge had a fan club called 'The Stackridge Rhubarb Thrashing Society'.) The rules? I managed to find one line:
Rhubarb Thrashing pits two blindfolded contestants against each other, who stand in dustbins and do battle with rhubarb.
Apparently rhubarb can do some damage, so I imagine one thrashes with the leaves. What I haven't managed to find is any indication of how the winners are declared... [Update: we have a new source that apparently clears things up.]