My issue with not using coils is that the air in the attic may come with lots of dust and insulation if you intend to do it directly. Also, where air comes in, it must go out....somewhere....which means you are going to exhaust heated air to the exterior which defeats the purpose.
If you have ducts in the attic and they are uninsulated, and don't want to put hydronic coils in the system, perhaps there is enough ductwork which could have aluminum heat exchangers screwed to it to absorb the heat from the attic, then a differential controller, like the RESOL could measure the air temp in the attic and on the main floor and vary the fan speed to move the heat down while keeping the necessary dT. This would keep the air within the building envelope.
Just a thought...
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