well now… i’ve got it. it’s surprisingly simple. but has caveats. the render target gotta be the same apect ratio as the viewport. and it’s lagging a frame or 2 behind. and… yes… it is rendering the whole mirror view, but only needs the mirror plane information, which should be rendered reverse stenciled in a pre pass and skip everything that is not part of the mirror surface. that is the optimization step todo.
viewport reprojection.
and the blueprint. ignore the debug mess. lol
sharing is caring. 
it’s not perfectly usable anyway. shruge