Not sure if it's the same thing, but at Kannagara dojo there used to be a year-end/beginning class which ended with midnight misogi. Now the traditional year-end/beginning jinja activities are in full effect though so I don't believe that happens any more. It sounds like your event is a bit more strenuous than what I recall...unless you count a winter dip in the Pilchuck river as strenuous.
No, we don't have water torture involved lol
Sounds like a blast though...hard to find a river like that in Florida though.