The radiator does not work mainly by radiating heat, it works mainly by convection, i.e. the air touching the radiator rises and circulates round the room. Although the chair back might get hot, the radiator should still do its job. If you could move the chair slightly forwards you might have room to put something between the chair and radiator; something like a fireguard with foil over it. I can't think of a simple way to do this. A handyman might be able to put together a vertical wooden support using slats of wood with little wooden feet at right angles to the radiator, and then foil could be draped over it (the wooden support) down the front and back with some adhesive tape holding the bottom edges together. I wouldn't recommend covering the radiator itself, as that would cut down the heat available to warm up the room.