How can you use both fontspec and xmpincl?

Question

Is there a way to use both the fontspec and the xmpincl packages? Using PDFTeX returns

Fatal fontspec error: "cannot-use-pdftex"

The fontspec package requires either XeTeX or LuaTeX to function.

You must change your typesetting engine to, e.g., "xelatex" or "lualatex"

XeLaTeX and LuaLaTeX returns

Package xmpincl Error: latex is not supported by the \includexmp package.

Answer 1

Excluding pdflatex, which is not an option since it cannot be used together with fontspec, let's try to understand whether xelatex or lualatex can be used.

The file xmpincl.sty contains the following lines:

\RequirePackage{ifpdf}
\ifpdf\else
\PackageWarningNoLine{xmpincl}%
  {Only pdflatex is supported by the xmpincl package}
\newcommand{\includexmp}[1]{%
  \PackageError{xmpincl}%
  {latex is not supported by the \protect\includexmp\space package}%
  {You tried to include XMP metadata in DVI production.\MessageBreak
   That doesn't work, and I friendly tried to warn you.\MessageBreak
   Just continue and pretend nothing is wrong,\MessageBreak
   but please remove the package or switch to pdflatex.}
}
\relax\expandafter\endinput
\fi

This means that it returns an error when you use \includexmp if the \ifpdf test is false.

You can use the following minimal file to test whether \ifpdf returns true or false

\documentclass{article}
\usepackage{ifpdf}
\begin{document}
\ifpdf
\texttt{xmpincl} can be used
\else
\texttt{xmpincl} cannot be used
\fi
\end{document} 

With lualatex (beta-0.76.0-2013062820) I get

while with xelatex (3.1415926-2.5-0.9999.3) I get

This means that with an up-to-date luatex engine you should be able to use fontspec together with xmpincl.