Just remember it is Renault that decides whether its an authorised warranty repair or not - not the dealer. If a clutch failure led to a bell housing repair - even Renault would acknowledge a repair is cheaper than a new casing and gearbox rebuild - there is no way any of the debris from this can have entered the gearbox.
The route will be - roadtest - yep syncro gone - gearbox out - cost of repair vs new, which is cheapest - new - put new box in. I do not believe they will strip the box to check the synchro, it will cost renault more to strip and make a decision to replace when you can tell the problem from the drivers seat.
It is a mechanical failure which is not consequential damage from a non covered part and is therefore covered.
Just tell them synchro is gone, test drive with service manager, get him to acknowledge the failure and get the repair authorised before they touch the car - no risk for you and the dealer gets paid and makes a profit on his parts and labour.
It costs Renault France in the end and believe me they aren't paying more than £500max for those boxes in the volumes they buy. They'll just wince at the dealer labour bill!