Isn't it the case that in some versions, Fantine only sells her hair, not her teeth as well? I thought the teeth thing came from the novel, not the musical.
(From what I've seen, when schools and the like perform it, the girl who plays Fantine usually wears a wig
In the book, she sells her hair and her front teeth. Don't know where the bit about the locket came from; I'm guessing it takes too long to properly apply toothblack on stage so they had to have her sell something else. Not sure why they changed it to her back teeth in the movie, unless it's too difficult to SFX out teeth. (Or, it'd be too obvious that her teeth aren't really gone if she's singing clearly.)
Generally, if the actress playing Fantine has short hair, they'll put her in a wig, have her go offstage during the appropriate verse in "Lovely Ladies" and remove the wig. If the actress has long hair, they'll tuck the hair under a bonnet.