Hello again,
I just discovered another weird problem with small caps when using some fonts, e.g. Adobe Jenson Pro or Adobe Garamond Pro.
When old-style figures are enabled, the code below generates "GEOff" instead of "GEOFF". Another ligature bug, it seems -- which is strange because I don't see why old-style figures should affect ligatures at all (not that I know anything about these things), and also because the second line should disable ligatures in small caps altogether.
\definefontfeature[smallcaps][smallcaps][liga=no]