Moving on from the previous suggestion re. pumping air into the radiator to evacuate the water.Why not have a spring loaded spigot on the radiator connection.As you disconnect the valve from the raidiator the spigot activates sealing the radiator connection and preventing the water in the radiator leaking out usually in the position that you can't usually get a container under. Cue carpet damage.You then lift the radiator from the wall and empty in a more convenient location

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