Display horizon in viewport?

Hello!
I’d like to see the horizon as a line in a viewport (for camera matching purposes - not during rendering!).

By turning on Ground Plane in any shaded display mode, it can be simulated. There’s a problem, though: but when looking at the horizon from below, the ground plane is always opaque, even if I edit it’s material’s transparency in the Ground Plane panel. This looks like a bug, because it makes no sense.

In a wireframe display mode, it seems not to be possible at all to see the horizon, is it?

Thanks!