Confirmed here too. In Hidden Line, i can gray classes in the viewports OIP and they gray the hidden line components in the grayed classes.
If I attempt to gray layers, then in Hidden Line the objects simply disappear, rather than graying as expected. Enabling "Project Screen Objects" on the viewport allows the objects to appear, but as you say, they are hollow wireframe, so you can see the geometry behind them rather than the expected HL appearance.
I'll test this a bit more to make sure I'm not missing something, but it looks like an issue that I'll need to submit.