Ok, for what it’s worth, I’ve found quick & dirty but actually well working workaround for my use case.
I choosed to simulate a flat map by compensating the heightmap accordingly to the WGS84 ellipsoid reference so that the sea altitude will always be 0 in Unreal coordinates. To achieve this, each point of the heightmap is adjusted by an offset corresponding to the height difference on the ellipsoid relatively to the center of the map.
Of course, the downside is that there will be some ‘walls’ at the edges of the map. The larger the map is, the highest walls will be. This solution is acceptable for a restricted portion of the world, but definitely not for a whole planet.