Get coordinates of max y position

How to get the maximum position of y axis that is still visible in the scene.

At which depth and with what units? At camera lever that’s camera.y +/- camera.up * 0.5, but that’s in UV, not in pixels.

I guess it is better to explain what I am trying to do.

I am moving the (perspective) camera around the scene and while the camera is moving and rotating I want a selected mesh to always stay on the point described in the question for a few seconds.

Basicly similar movment to a HUD, but I cannot use HUD.