alternatively, for loading to ThreeJS, I could convert the brep object to a mesh and then save the file with mesh objects.
https://developer.rhino3d.com/api/rhinocommon/rhino.geometry.mesh/createfrombrep
...
mesh_part = Rhino.Geometry.Mesh.CreateFromBrep(part, meshingParameters)
rh_file.Objects.AddMesh(mesh_part , attributes)