According to an email flyer I received yesterday from French Cycling Holidays:
"Following the accident suffered by Fran Ventoso in the Paris-Roubaix race, purportedly due to being cut by a disc brake, the French Federation has, in its wisdom, decided to outlaw discs in all FFC sanctioned events.
This includes sportives like La Marmotte and the Etape du Tour, as both are run under FFC license. We have spoken to ASO and Sportcommunication who run these events, but they have so far not issued a definitive stance; however they have indicated that whilst not in favour of the ruling they may be forced to comply."
If correct, maybe something to bear in mind when choosing which bike to take to certain French events.