I've played for 2 clubs who have done it, was a great event...
12.00 KO (so the boys who wanted to go hit the welsh derbies could), 2 'elected' coaches would select 2 teams (usually 25 men squads lol), rolling subs on and off.
There would be an obligatory port at kick off, yellows would be a lap of the pitch, reds would be a naked lap.
They would play 2 halves of 40, but there would be a water bottle filled with port for both teams, you wanted a water you had to swig port first lol.
Last time I played was 2015, ref scored a hat trick of tries for the 'home' team in the first half, so they made the 70kg 60 year old play tight head in the 2nd half.
Can't remember the score, I remember vomiting during my yellow card, and somehow a few of the youth boys had snuk into the game and starting scoring worldies from 50m out, so 2 got tied to the posts to keep them busy hahaha
Great memories, sadly I heard the 1 club never restarted the tradition after that, there was a complaint by a mother, about the goings on in the club involving her 23 year old son who had got drunk and told her all the fun.
They now only run a 1st team, lost the 2nds and 3rds, vets vanished and barely have a junior section, the junior section they do have is toxic, mums arguing with each other, virtually every junior coach having complaints about them favouring players etc.
I heard recently that the junior teams had to rotate players in every position and everyone had to have equal game time... what a way to kill a sports club!