Now it can be told!
With the Brawl over (for the PCs anyhow), I can reveal the source from which I swiped the whole scenario: It's The Great Outdoor Fight, a beloved storyline from Chris Onstad's brilliant webcomic Achewood.
Outdoor battle royale in a huge fenced-in arena? Check.
Rag-tag band of followers? Check.
Recumbent tai chi? Clearing off the wounded by crane? Check.
The whole tale can be found online, starting here (well, there's a prelude of sorts featuring a junkie squirrel and anatomically correct mobile phone accessories). Also available in a print edition published by Dark Horse Comics. Highly recommended!