As I mentioned, spawning is not feasible because these things happen at editor time in the construction script.
I also described in the OP that my workaround basically does exactly that, except using “add component” - but it’s an unnecessary and not exactly cheap step to do every time the actor updates, which, when you drag around spline points, is supposed to happen per frame. Especially since the component serves no other function, because the actual system uses spline mesh components, not static mesh components. When you do that to dozens of meshes every frame, it creates some overhead that really shouldn’t be necessary. 