Draw a symbol into the fixture family using Model Lines, and make them the only thing that displays at the Course detail setting (or some other choice like that that suits your usage). This will work in 3D also. But it will retain it's physical size, so zooming might be problematic.
...or...
Use Symbolic Lines instead, or nest in an actual Symbol family, and make THAT the only thing that shows up in Course detail setting or whatever. Will auto-scale nicely, but will only show up in views normal to the placement plane of the Symbolic Lines or nested Symbol family. For an example of how this works, load and place one of the MEP Electrical Device families, such as a receptacle or a light switch.
