One option that I passed over initially as a "gotta get this out now" solution was to generate a "custom day" (SizingPeriod:DesignDay object) in E+ for my 3/21 equinox using the ASHRAEClearSky Solar Model Indicator and a Sky Clearness of 1.0 (1.2 is actually the max.) I believe this would be a very simple task for measure writing in OS.

There are clear disadvantages when using E+ (vs Radiance) for daylighting studies as Rob as explained. But for a quick LEED compliance report...?

Is it fair to say this seems to be a case of simple tools for simple answers? (LEED not daylighting studies.)

I still think Rob has the right solution in integrating this into OS/Rad, btw.