The legal possibility may exist, but would the equipment still work, does the software need updating since the 2020 versions, and does the building still meet all the other relevant certification for passenger use? Opening a side gate and showing your passport to a rather bored PAF officer is perfectly feasible though.
On mobile, so apologies for the formatting. According to the latest list of French border crossings:
Land borders
(1) Bourg Saint Maurice railway station (open from the beginning of December to mid-April)
(2)Moûtiers railway station (open from the beginning of December to mid-April)
(3)Ashford International railway station
(4)Cheriton/Coquelles
(5)Chessy-Marne-la-Vallée railway station
(6)Fréthun railway station
(7)Lille-Europe railway station
(8)Paris-Nord railway station
(9)St-Pancras railway station
(10)Ebbsfleet railway station
(11)Pas de la Case-Porta
(12)Roissy TGV railway station - airport
If they are listed here, then they are available for use. I don't think there would be any major drama in reactivating Calais-Fréthun for temporary purposes, as it's likely that there's only small issues with the technical infrastructure.
The bigger issue will come later - as it's not clear to me if France will equip the unused border crossings with the EES equipment.
I'd also point out - at least in France, they have many "on demand" border crossings at small regional airports. There's no reason why they couldn't also carry out controls, especially as the French authorise their customs officers to carry out immigration controls.