The first thing, this is not a law. It is some agreement between unions or something like that and employers. The agreement itself is not about employees, it is about contractors. Contractors in France do not have 35 hours a week work time limit, they can work as much hours as they want.

Also, the agreement does not says they can not check mails after some hour, the employer must give them possibility to disconnect for 12 consecutive hours. You can expect them to be available for remaining 12 hours.

So, all you need is either employee on call or two contractor to switch turn-offable times.