I am also very much interested by a flat terrain.
I am looking for a terrain engine to fill the gap between Unreal landscape (limited in size) and Cesium for Unreal (unlimited). I am rendering a terrain of ~80km2.
What I do to simulate a flat terrain with Cesium for Unreal is I compensate 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.
Cf. I cannot have a consistent z=0 level - #8 by 0x974
This is a quick & dirty hack but it works.
That allow me to use for example Oceanology with Cesium for Unreal.