The RhinoObject.GetRenderMeshes function creates a mesh for the SubDs, even if it already exists.
it happens once, after that it works like with other objects.
here i get the mesh under the mouse with
var objs = new Rhino.DocObjects.RhinoObject[] { myObject };
Rhino.DocObjects.RhinoObject.GetRenderMeshes(objs , true, false)