Why is \scshape not changing in XeLaTeX?

Question

I'm trying to alter \scshape in XeLaTeX, but for some reason, three reasonable attempts (xpatch, \def, and \renewcommand) seem to have no effect whatsoever (only in XeLaTeX). A nonworking example is below (note that the 5 em space is just to make it clear when the redefinition is working, and obviously not intended for use in a real document):

\documentclass{article}
\usepackage{xpatch}
\xapptocmd{\scshape}
  {\spaceskip=5em}
  {}{}
%\let\myscshape\scshape
%\def\scshape{\spaceskip=5em\myscshape}
%\renewcommand{\scshape}{\myscshape\spaceskip=5em}



\begin{document}
\noindent\textsc{Test thing test}\\
\scshape Test thing test\\
\scshape\spaceskip=5em Test thing test
\end{document}

Answer 1

Do the patch at begin document.

\documentclass{article}
\usepackage{xpatch}
\usepackage{fontspec}

\AtBeginDocument{%
\xapptocmd{\scshape}
  {\spaceskip=5em}
  {}{}%
}

\begin{document}

\textsc{Test thing test}

\scshape Test thing test

\end{document}