Welcome to the forum.
Honestly - its great to see such enthusiasm! I am going to be very very cheeky (but positive I hope) - you (and others) may hate me for this- I'm going to repost your post with some paragraphs that us non LeMonde readers will find easier - apologies. When I post in your neck of the woods pleas feel free to improve a 80%+ post into an excellent post (or slap me in the face!)
I ride with an group that does 'normal XC' night rides every week. We do 'Alleying' urban rides several times each year.
The format of Alleying is a well organised and pre-planned 20-30 mile urban night ride, at tempo, using alleys, service roads, car parks, precincts, wasteland, subways, parks, church yards, multi-storey car parks, etc.
The routes are circular, much like XC rides, but without any loose surfaces. Optimum conditions are a dry and warm night, starting at twilight.
Roads are avoided, and only used where absolutely necessary to link 'trails'. It's fast and furious riding, with constant hard accelerating and braking. Your wits definitely need to be about you, otherwise surface changes, kerbs, bollards and other street furniture could spell disaster.
We work blocks of flats and supermarket foyers into the mix - which is all very naughty!
Steps, banks, drops and any other fun features are included where possible as 'bottle tests' for the following group.
6-10 evenly matched riders is best to maintain a good speed and uninterrupted flow. The weapon of choice is a rigid steel singlespeed mtb, with slick tyres and a 55" gear.
Intimate local knowledge is vital, and the leader uses this to plan the route carefully in advance. Our area is blessed with miles of alleys around housing estates, and plenty of rear service roads behind terraces of houses.
There are also plenty of pedestrian precincts and other hard surfaced thoroughfares that are not roads. Putting together a decent 30 mile route is no problem. We've developed an entire faux lore and tradition of Alleying in the five years since we started it.
The language of alleying is French (for the leader's commands and warnings), and pseudo rules and conventions are constantly hinted at to bemuse newcomers and amuse regulars - a bit like the 'Mornington Crescent' game in Radio 4's 'I'm Sorry I Haven't a Clue'.
Alleying is the best fun you'll ever have on two wheels and would make a great mag article. PM me if you want to visit and experience the thrill.