I have no direct experience, but it looks like a clear case to me:
As long as there is no loss of coolant there is no problem.
Engine cooling does not rely on heat radiation by the heater (if so, all engines would overheat in summer). I would proceed. If the leak is stopped by disabling the pump (there might still be coolant flow in the heater circuit due to convection) I would not worry.
Good luck, Bruno

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1992 300TE 160 kmiles