Bad idea. That adds pressure to the radiator inlet side, and removes some of the hot water needed to heat the cool side of the block. The outlet in the block can be used to supply a heater core, if cabin heat is needed. The outlet of the core if used, would be plumbed to the outlet tank of the radiator near the return or suction side outlet to the pump. A simple car type cable operated valve can be used to control water flow to the core or cut it off to zero flow.

Lynn E. Hanover

My radiator has in inlet top and an outlet bottom and on opposite side. The water pump outlet goes to the top radiator and I welded an extra 5/8 hose connector there to receive hot water from what you describe as the midpoint of the flow path. Good idea or bad?

Thanks