Eric, I imagine that you have a direct contact with someone organising LSdGdM.
I submitted a question on their website but didn't get a reply. No rush, but could you ask them if La Douane/Police aux Frontières will have a presence in La Golfe for boats arriving from abroad please?
Boats coming from England obviously have to clear customs but may do so in perhaps Roscoff unless sailing non-stop to Morbihan, in which case it seems that like a boat coming from the west (Ireland) you would have to call in to Brest or Lorient (Frontier Ports) to clear customs. Surely there are tall-ships arriving direct to Morbihan that will require customs clearance on arrival. A boat coming from Ireland is supposed to notify immigration as Ireland is outside the Schengen Area, and in my case, if I get there, I'm a British Citizen so I'm in the same boat as the other Brits and must get my passport stamped. What a faff!