There should be a bDisallowNanite flag (which can be set a runtime) on StaticMeshComponent for the renderer to fallback to the nanite proxy mesh if r.Nanite.ProxyRenderMode is set to 0 which should be the default.
Did you watch the State of Unreal talks held at the GDC recently? Epic explained how they updated Fortnite assets to work with Nanite. Trees and grass for example was updated so that individual leaves were modelled and the material didn’t need to be masked anymore. Of course if the mask comes from user input that makes this more complicated. If you’re interested in the details you can read more here: Bringing Nanite to Fortnite Battle Royale in Chapter 4 - Unreal Engine