Why not do some kind of Compass ride? encourage people to jump on where they like at set times.
Start Trafalgar Square at 9pm, head to South beers pub for 10pm quick pint or whatever, then to Easts to pick up some hipster twats with yellow bikes at 11pm, Norths for midnight up the big bloody hill (maybe race it as roads will be quiet as a mouse), West beers for kebabs at the meat pit around 1am then bridge ride it back to Tower Bridge and call endex around half 2.
It'll be fucking cold as a fucking mother-fucker though so no waiting around, see the caravan and jump on, you get lost and can't find your way home; ask a Policeman or call a cab/your mum...
Really like this idea.