ANSI - Exposure Guide's first adjustment for clouds is with a haze over the sun. That was basically the same thing I found in Jones' paper (which the standard was based). It appears that an unobscured sun with or without clouds surrouding it has approximately the same illuminance.

BTW, the ISO speed standard's color temperature exposure changed in 1960 from skylight to daylight. This prompted a small change in the proposed speed constant (adjust for slower resulting speeds).