Barish,
I understand what you're saying about the temperature of the radiator feed water, but I think you could probably do it by using a PI loop to determine how much heat is required, rather than the outdoor temperature. With a PI algorithm, you can decide how much heat to add by measuring how far you are from the desired temperature and how long you have not been at the desired temperature. The outdoor temperature would affect how fast you drift from the desired temperature, but you would probably need to determine experimentally how much gain to add based on the outdoor temperature. A PI would do this automatically, and without the need for the outdoor temp. Food for thought.