You would need to have 2 temp sensors, one for the attic and one in the return air duct below to determine if there is enough heat capacity in the attic. A RESOL solar controller could do this and run the pump and fan motor (if there is one as some of the old systems were gravity circulation). It would have to be variable speed pumping to keep any deltaT high enough so as to not make the air feel too cold. It is doable. You could use an Arduino control if you are electronically handy.
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