You may not depart on a flight, or flight pairing, while you know you'll be outside of FDP AND having the means at your disposal NOT to exceed FDP (home base have standby/replacement crew).
Sorry mate. Please find me something in the law that states that.....
As said by Avi8tor, the Co would be stupid to allow a possible unplanned stop for a/c and crew, but there is nada in the law that prohibits you from leaving - in fact, that is the whole intent of extension.