Now this is an interesting idea.
I say it's as simple as having France regain some of their colonies, or gain a few more. Have a bad couple of years in the south to provide incentive for Occitan speakers to move to the colonies, get to the point in French history when they start waging linguistic war on the "inferior dialects", then come up with some kind of political crisis that leads to one of their American colonies declaring independence, and have them adopt Occitan as the official language out of spite (kind of how the USA in its early days was toying with the idea of adopting German as the official language to piss off the British even more).