Rib Cross Section / Section Omission

Build it as an assembly how you would build it. There you can exclude your rip.

You could also use the model's geometry and create a drawing sketch over the top of the model.

once you created the drafting geometry, you can create a hatch pattern in the closed section. Use hidden line view to get all the geometry needed from the "master model" for the operation.

This really is an odd feature of the standard. It makes sense to make thing clear, but it is odd. I don't suspect PTC will try to do this as a standard.

There are a few other ways to get there as a single part. Having overlaying views comes to mind where one has the rib and the other does not. You can turn off hatch patterns in sections, and you can hide feature edges. It is all a very manual means to an end.