Nope... doesn't seem possible...

Even if you get the lines to draw on the screen it will erase each time the screen is refreshed... unless you keep drawing them over and over again.

Even if you do that, and you wish to use it with HalfLife, it still wouldn't work because HalfLife uses DirectX rendering which deals straight with the hardware and skips the Operating System GDI (graphic design interface). So the lines will not show up when HalfLife is maximized.

Unless there is some other way, in which I don't know about.