Defining italic small caps globally using fontspec in LuaLaTeX

Question

I am using an OTF font that has small caps but not italic or slanted small caps, and there are a smattering of places throughout a large document I'm working on where italic small caps are needed. I know I can do it on a one-by-one basis with a \newcommand but I was hoping to avoid needing to go back through the whole document and find all the cases where it's not compiling correctly and just define the fake italic small caps as the shape for the whole document. I tried the following, but it didn't produce the effect:

\setmainfont{Sabon LT Std}[
     Numbers=OldStyle,
     Ligatures=Common,
     ItalicFeatures={
          SmallCapsFeatures = {FakeSlant=0.2}}]

Any suggestions?

Edit: The font Sabon LT Std has small caps in the roman but not in the italics, nor any of the others.

\documentclass{minimal}
\usepackage{fontspec}
\setmainfont{Sabon LT Std}[
    UprightFont = *-roman,
    BoldFont = *-bold,
    ItalicFont = *-italic,
    BoldItalicFont = *-bolditalic,
    Numbers=OldStyle,
    Ligatures=Common]   
\begin{document}
\textsc{Small caps}
\textit{Italics}
\textit{\textsc{Italic small caps}}
\end{document}

Answer 1

I don't have your font, but this here works fine for me

\documentclass{article}
\usepackage{fontspec}
\setmainfont[
    ItalicFeatures =
    {
     SmallCapsFont={Arial},
     SmallCapsFeatures={FakeSlant=-0.2,RawFeature={+smcp}}
    },
     BoldItalicFeatures={
     SmallCapsFont={Arial/B},
     SmallCapsFeatures={FakeSlant=-0.2,RawFeature={+smcp}}
    }
    ]{TeX Gyre Termes}
\begin{document}
normal
\textsc{Small caps}
\textit{Italics}
\textit{\textsc{Italic small caps}}

\textit{\textsc{Italic \bfseries small caps}}

\textbf{\textit{Some \textsc{Italic small caps}}}
\end{document}