Might be worthwhile investigating the Citroen Car Club and/or 2CVGB. I do know that discounted insurance is available even for converted vehicles via the clubs. They do like you to join though.

Compared to an H Van camper van conversion (the cuboid corrugated iron device once seen at every French market) I would have thought that converting and insuring a Berlingo would be a doddle! And you don't have to worry about obsolete tyre sizes, 3 speed back to front gearbox, deckchair driving seat, fwd with no power steering, 70 kph top speed and similar Gallic charms.

Course you could go completely left field and convert a Deux Chevaux van....