Les Mis isn't too susceptible to stunt casting because it is largely an ensemble show, and the handful of lead roles are simply too difficult vocally for most non-singers to handle. This would limit them to pop music personalities, as they've already done a bit of with Debbie Gibson as Eponine and Ricky Martin as Marius. Those are 2 roles where you can pass off a pop voice, so long as they have a decent range. Also Monsieur and Madame T could be played by any set of comedic actors with decent musical ability. The majority of the other featured parts are reliant on having capable "legit" or operatic singers, otherwise why bother doing it?
The other thing with Les Mis is that it doesn't leave much room for a performer to add anything "cute" or take liberties to make themselves standout. It is a historical piece that is very demanding of every cast member and basically needs to be done a certain way. This wouldn't make it a very attractive choice for too many of these famous people who want to dabble in Broadway. Sure a few vocally-adept popstars might slip through the cracks, and if they can pull it off and are willing to work that hard, good for them. But Les Mis is not for the faint of heart, not for the Hasselhoffs and Fatones of the world.
"The last train out of any station will not be full of nice guys."
- Dr. Hunter S. Thompson
"I wash my face, then drink beer, then I weep.
Say a prayer and induce insincere self-abuse,
till I'm fast asleep"- In Trousers
