This is something I've thought of as well. You would need two ducts,
supply and return, otherwise you'd draw air into the attic from the
outside, which would make the whole thing useless in the winter. Using
a filter over the intake in the attic as someone else mentioned is a
necessity. It must be pleated hepa grade to ensure no attic
contaminants move thru the house. Bigger is better than smaller- lower
resistance, longer maintenance cycle. A bit more $ up front, but can be
vacuumed clean many times with care. Use a fan/blower that's just large
enough to move the air at a reasonable rate. You'll have to figure out
for yourself how to control the on-off cycle. If you have predictable
sun in winter, a photo switch might do, or a timer or ? otherwise.
There should also be a filter on the return in the attic, as there may
be times/conditions when the fan is off that you get air going down
that way. All of this depends on if/how well your attic is ventilated
and other "local" conditions.