I spoke with one of our main BP and material guys today and he didn’t know of a way to light up individual polygons this way. He did say if you were using BPs you could find a way to do some really complicated math that would be way more complex than it’s worth to achieve this. He was unsure if it would work properly or not though.
His suggestion was that if you were comfortable with C++ you could write a function that would check your player position and somehow tie that into your mesh to see what polygon it is on. You would have to keep in mind that even though your mesh would be quads when you create it, the engine is going to triangulate this mesh so you would have to calculate for two polygons during this.
His initial suggestion, if it’s a simple enough mesh, would be to lay out your mesh and any pieces you want to light up be a separate mesh that would switch materials when player makes contact.
I hope this helps you out!
Keep up the great work with your project!