I have well knoweledge of NDI since im doing a lot of A/V work now, AFAIK theres no ndi integration on lua, but doing it as a plugin should be doable using printwindow api to get a bitmap and pass to ndi, but we have to mind that windows are not videos, they usually change size, minimize, etc. This could lead to invalid video on any resize, or impossibility to capture if minimized, with buffer underflow. So it will work only on the desirable circunstances...
If youre interested i can throw some bit of plugin to test