How does Font Book.app determine the "Design style" for a Smart Collection?
OK, I think I figured it out...  Opening a font in RoboFont, I scoured all the obscure meta-info available and finally found the entry that seems to correspond to what FontBook uses for the Design Style in the section called Panose (a term I had never heard of!).  Long story short... FontBook seems to probably use the Proportion to determine the style.