In June 2013 we ran a one day editathon at the Royal Opera House in London. The aim was to improve our content regarding ballet, and we had an assortment of Wikipedians, ballet fans and Royal Opera House people - including their archivist. Apart from those of us who were ballet fans, and we did have a couple of wikipedians who edit ballet and we don't otherwise see at GLAM events, we'd chosen the subject area because writing articles about pre internet cultural events helps address some of our institutional bias. The Archivist brought collections of press cuttings and other source material and we were shown a video of 70s ballet.
Learning from past experience, we minimised the too many cooks edit conflict problems by having half a dozen tables and spreading ourselves across as many articles. That worked well, but none of the articles got particularly advanced on the day.