@unseenphil replied:
It really depends on what aspect of the source material you want to focus on.
For example, if you’re keen on the game of cat-and-mouse between quasi-heroic criminal protagonists and the Law, you might be able to do something with Avery Alder’s Perfect Unrevised. It’d be a total setting reskin, but the underlying system has much to recommend it for that sort of play. In brief, it has a rotating-GM setup where each player is assigned the role of the Law with respect to another player’s criminal; when your character takes centre stage, your assigned Law player steps into the GM role to try and bring you down. In this way, everybody gets a chance to be someone else’s personal Javert. Heck, in a slightly tongue-in-cheek game, the Law players could literally pass around the role of Javert!
On the other hand, if you’d prefer to zoom out and engage with the political big picture and the sweep of history – rambling asides about the Battle of Waterloo included! – you might have a look at Kingdom. It’s a narrative game about power dynamics within communities, with the traditional GM position subdivided among multiple roles; for example, a player in the Power role gets to decide what the community does, but a Perspective player decides what the material consequences of those actions are, and a Touchstone player decides what the people want and how they respond. And yes, there are rules for usurping other players’ roles if you don’t like how things are going down.
On the mutant third hand, if you want to keep things personal, but you could do without the mechanically enforced interrogation of power hierarchies – maybe you just want a breezy game about people getting entangled in inadvisable relationships, singing about their feelings, then dying horribly – I’d be inclined to knock together a suitable Fiasco playset. I’m actually kind of surprised there isn’t one already, but I couldn’t seem to find anything in the official repository; perhaps you’ll have better luck than I did!
Finally, in terms of upcoming games to keep an eye on, there’s Gears of Defiance, which successfully Kickstarted back in April and will in theory be out before the end of the year. This one has some fun conceits, including an honest-to-goodness alignment grid whose axes are Traditionalism/Radicalism and Liberty/Authority rather than Law/Chaos and Good/Evil. A 50-page ashcan preview is available here, if you willing to pay three bucks for a teaser.
So wait, which one of these is the best for rambling asides about the history of monasteries and the Paris sewer system?
Possibly Microscope? A self-described “game of fractal histories”, it alternates between very zoomed-out history building play and zoomed-in character scenes where the players explore pivotal moments in that history. There’s no built-in expectation of continuity or even linearity in the character scenes, so you can easily jump in and play out the day in the life of a character who has never appeared before and will never appear again just to contextualise some event the character in question was present for. It typically focuses on a somewhat broader historical scope than the events of Les Mis, so it’d need some tweaking, but I think it could be made to work.
Of course, another potential issue with Microscope is that there’s a built-in assumption that those zoomed-in asides have a. identifiable characters, and b. a point. If you really want to play up the sheer aimlessness of some of the source material’s asides, you might be able to work something out with a heavily modified version of The Extraordinary Adventures of Baron Munchausen; other players having the ability to interject their own bizarre complications and nitpicking objections would help to sustain interest in what would otherwise be a series of long, pointless improvisational speeches.