Setting Blueprint Interface Variable in Instance

I have a set up a blueprint interface (called PCG_INode) with just one method: Generate. This method returns a struct that is used to generate static mesh instances in the construction script of another blueprint (called PCG_Instance). The PCG_Instance blueprint has an editable variable called RootNode of type PCG_INode. I have a few blueprints that implement the PCG_INode interface’s generate function. The idea was that I could drag a PCG_Instance into the editor, and then set its RootNode to an instance of PCG_INode through the details panel, and depending on which blueprint I select, it will generate different things.

The RootNode variable appears in the details panel, and looks a lot like it’s going to let me create an instance by selecting it in the content browser. However, its value is None and I am unable to select any of my PCG_INode implementors. I’ve included a screenshot below (open in new tab for full resolution):

Any ideas on how to proceed? Is what I’m trying to do even possible?