Success!
I used “Get Hit Result Under Cursor By Channel” in the PlayerController BP, and then just a few branches to decide whether to highlight or not, or to de-highlight previous selections.
In the end, I just had to learn a lot more about what kinds of functions are available in UE4! Hopefully this helps someone else in the future!
Event to select piece
Decisions to highlight or not, or de-highlight previous selection, if there is one