Hi Ian, This is not a situation I've come across, or indeed PVC used internally. So I can only make broad comments. You ask if it's safe, at the temperature of a water filled radiator, there cannot be a fire risk. PVC expands a lot with temp though, so I would expect some distortion. Could I suggest fixing a false panel to the cladding first, the same size as the radiator. MDF or ply maybe, get it painted, then fix your rad brackets to the panel.