Use French when no translation
Yes, the Occitanie is mainly in France, so occitan speakers speak French with a high probability, so it could be logical to fallback to French.
I think it's a bad idea. Of course, Occitania is mainly in France and most Occitan people speak French (an no Occitan at all) but, on Occitan wikipedia, a large part of users come from Catalonia and they don't necessarily speak French. A bit more difficult problem is also the historical heritage of the relationships between France and Occitania as France and French are widely considered as the main responsibles for the disappearance of Occitan language. So, I think seeing French on Occitan wikipedia will create harsh reactions against.
Good point. However, according to https://stats.wikimedia.org/wikimedia/squids/SquidReportPageViewsPerLanguageBreakdown.htm#Occitan , it seems that the Occitan Wikipedia receives less than 6 % of visits from Catalonia/Spain. About 5 % of edits come from Spain: https://web.archive.org/web/20150426123709/http://stats.wikimedia.org/wikimedia/squids/SquidReportPageEditsPerLanguageBreakdown.htm