Duster,

Night and IMC are independent of each other. Instrument Meterological Conditions are driven by ceiling and visibility, and have no regulatory influence by the position of the sun. (Meteorological influences are a different consideration however I doubt there will be many places where the short duration of the solar eclipse will change the weather conditions from VMC to IMC)

Example: A Class D airport is not considered IMC at night, and a pilot is not required to fly uner IFR into a Class D airport at night, unless the ceiling or visibility drop below VFR minimums.

Unless properly equipped and certified, gliders are not authorized to fly at night or in IMC, however I maintain that a solar eclipse does not for the definition of either condition. As long as you stay out of the clouds and maintain the minimum visibility for your class of airspace you are good to go.

-SW