It was great to hear so much of his music in the film, and I hope that more people get a chance to see it. The movie conflates together a number of different historical figures and events, but the general outline of the story it tells is more or less true.
Together with the opera singer and director Lorraine Goodman, I wrote a stage play called God of Arms, which is also based on the life of the Chevalier de Saint-Georges. We even had a series of readings of the piece, one featuring the incomparable Norm Lewis, but we never found a producer who was interested.
In the 1770s, Saint-Georges made a bid to take over the Paris Opera. Unfortunately, three prima donnas wrote a petition that he not be placed in charge of them, and the opera was instead turned over to the courtiers in charge of the king's amusements. Still, Saint-Georges kept composing.
His first opera, Ernestine, had a libretto by Pierre Choderlos de Laclos, best known today as the author of the novel Les Liaisons dangereuses. Under the patronage of the Marquise de Montesson, Saint-Gearges wrote further operas, including L'amant anonyme, which has survived intact.
It's a pity you don't see opera companies today staging L'amant anonyme. It's also a pity theatres don't want to stage God of Arms! If you're interested in reading the play, please contact me.