I did some fiddling around and managed to accomplish piece selection from the fracture editor list. This solves one of the issues that I encountered.
I also managed to get the assigned materials to show up. The viewport just wasn’t reflecting the changes until the object was clicked on again (I guess clicking an object refreshes or reaquires its attributes?)
The remaining issue (and most pressing one) is that the interior materials wont assign to the interior faces. I’ve tried to set the index at the moment of fracture, and in the fracture material editor, but it doesn’t seem to have any effect.