It's not particularly elegant but I've seen at least one main board with rear DP inputs to allow routing discrete GPU video to the system Thunderbolt controller

It's not particularly elegant but I've seen at least one main board with rear DP inputs to allow routing discrete GPU video to the system Thunderbolt controller. It's hard to believe that solution exists but not either (a) TB-equipped graphics cards or (b) internal DP ports to facilitate a system TB controller.