Because you are using an 11-speed Campy chain, and are willing to lose some gear range if you have a mechanical, I'd recommend you carry 1 or 2 spare pins for your chain and a good chain tool. You're not going to be able to carry a shop-quality chain tool on your bike, but this folding one by Park is a good choice.
If your chain breaks during a ride you will need to remove a link and reconnect with the new pin. Assuming your chain was properly sized to begin with, with one (or more) link removed you'll be able to use fewer of the bigger cogs of your cassette in combination with your big ring, and possibly your small ring.
That said, the advice to check your chain regularly is a good one. I use a Park tool for that as it is the most accurate, and easiest of the chain checkers I've used. Rather than always replacing chain and cassette together, there are cassette checkers as well, or you can tell if the cassette is worn if a new chain doesn't go on and off the cogs smoothly.
I've ridden many thousands of miles on randonnees/brevets and for chain repair, only carry a chain tool and Wippermann Connex link. I've have had only one chain failure. That was 30 miles into LEL. It was caused because the chain got jammed during transit. To untangle it I had to break the chain with the only chain tool available. (I'd packed a different tool than I normally carry and hadn't checked it before I left.) The chain tool damaged the link so the new pin worked itself out after a short distance. The Wippermannn link saved my ride. If Wippermann manufactures a Connex link in the future that is compatible with 11-speed Campy, I'd recommend you carry that rather than a spare pin. It is also useful if you're packing your bike for flying.
As far as Campy on brevets, I personally ride Shimano, but know several randonneurs more experienced than I who use Campy. The best thing you can do is keep your bike well-maintained and "figure it out" if something goes wrong. You can't carry enough stuff to fix anything that can go wrong, and there may be no bike shops on the route at all. Improvising is part of the fun of randonneuring!