Where are the mirrors or the asteroid dust going to go? In orbit, obviously, but what sort of an orbit?
A geostationary or geosynchronous orbit is probably too useful for satellites to use. Any more space junk would probably not be a good idea for those vital routes.
What about a Saturn-type ring, then? Where would the “shadow” of the ring fall? I suppose that depends on the inclination of the ring to the solar orbital plane. We already have an inclination of 23°, giving us our seasons. What would happen if we aligned it with the equator? Either above or below the 35,786 kilometre height of the present geostationary orbit. If it was below, we could allow the orbit to decay to burn up in the atmosphere if we wanted it to. If it was above – could we give it a velocity that would slowly expand further and further away to prevent it being permanent ?
But instead of a flat ring we could have flat mats of reflectors in various different orbits allowing us to “tune” the effect they would have. If some had an intersecting point above an area we wanted to cool, we could intensify the cooling in that area by having the orbits pass above that particular area.