You have to look from the inside where the post is. For example, one of them is accessed behind the waste cover in the bathroom. I’ve heard from others that in some cases you need to remove some of the cabinetry molding.
Yes, I can't say it was easy, but on the 331 they were generally "accessible" through either some movable headliner material, or removable panels, inside cabinets and such. There was some cursing involved.
I've removed headliner, but it seems that there is an internal and external tub. The stanchions are mounted through the deck and into the external tub. The problem is that the internal tub, even with panels removed, does not give access to the external tub. I've reached out to Beneteau and have received no answer. I'll keep pushing and share any answer I get.