Create a 3D object for the plumbing fixture, which can be a simple cylindrical extrusion with a pipe connector on the end. Set the 3D object to not be visible in plan view. Then create an annotation symbol and load it into your plumbing fixture family. Place the annotation at the origin of the fixture and at the proper rotation. Then when you place the fixture in plan view of a model, you will only see the annotation symbol but you will have the ability to connect into it with a pipe. It's the same way an electrical wall device such as a receptacle, light switch, etc. is set up.