Hi @Simstad,
Unfortunately there’s no easy and direct way to do what you describe. It is possible to download a “clip” of an area of Cesium OSM Buildings using the Cesium Clips feature. And from there it is theoretically possible to add and modify the metadata included in the clipped model. Actually doing so, however, will require understanding the 3D Tiles and glTF specs quite well and writing some code to manipulate them.
Perhaps there’s a way to accomplish your goal without modifying the metadata embedded in Cesium OSM Buildings? For example, you could supply a custom material that pulls your custom data from somewhere, such as a texture you supply. The trick will be in looking up the appropriate values. You might be able to do that using the feature IDs embedded in Cesium OSM Buildings itself.
Another possibility to embed your custom data in a raster overlay, served up via WMS or another raster overlay type. Then Cesium for Unreal can drape it over the top of Cesium OSM Buildings, and a material can either use the colors from the overlay directly, or compute material parameters based on them.