Could be done but I'm warning you. I am a grammar Nazi.
But if he supports the Legitimists he'll get Spain as an ally. It isn't much but France faced with this Triple Alliance and Britain unwilling to commit yet will want every ally he can get. With French help the Spanish might become decent military wise.
How would the Dreyfus Affair go in TTL France?
I'm guessing that if Zola runs with J'Accuse he gets a nice comfy cell in prison.
We could see the Ligue des Patriotes become reorganized into a secret police with Boulanger in power, something like La Garde Patriote sounds like a good name.
Other thoughts on France- after the defeat of WWI could we see parts of the country fall under Communist control and parts under Royalist/right wing and a subsequent French Civl War?