Concerning the gendarmerie or the Court, you must present yourself during the working hours. However, you can contact the local gendarmerie brigade or police station to try to get a late appointment
https://www.interieur.gouv.fr/Contact/Contacter-une-brigade-de-gendarmerie-ou-un-commissariat-de-police. To save time, you should pre-fill and print the proxy document by clicking on the following link:
https://www.interieur.gouv.fr/Elections/Comment-voter/Le-vote-par-procuration