Rigify rigs rely on their rig_ui.py file to generate the addon's interface. If you link a character collection into a new file and proxy the rig, the UI does not generate because the rig_ui.py file is not linked. You need to append that text file as well.
In 2.79, you didn't need to do this. There was a setup using the Logic Controller that somehow made it so when you linked a group with a rigify rig, it also brought in the ui script. I don't know the details of how it worked, but it was part of the standard rigify setup, not something you needed to make yourself. This no longer works in 2.8 because the Logic Controller has been removed (it was part of the old Game Engine that has been removed.)
So, is there any way in 2.8 to get the same behavior again?