Rather than cut off the coolant flow through the matrix, which might cause unexpected coolant sensor issues, it would be safer to have a by pass between the input and output pipes. I'm visualising two T Pieces, with a tap on the by pass. So tap closed is as it is now, top open would allow coolant to by pass the matrix, although a second tap on the inlet pipe would ensure no flow completely. It's certainly do-able. Might even be worth it without AC.
