Voici une solution. Utilisant : `\reflectbox` de `graphicx` et `\AtBeginShipoutNext` `\AtBeginShipoutBox` de `atbegshi`
\documentclass{article}
\usepackage{lipsum}
\usepackage{atbegshi}
\usepackage{graphicx}
\begin{document}
\section{Foo}
\lipsum[1-3]
\section{Bar}
\lipsum[1-3]
\section{Baz}
\AtBeginShipoutNext{\reflectbox{\box\AtBeginShipoutBox}}
\lipsum[1-3]
\end{document}
**Edit** Suite au commentaires voici une mise à jour.
\documentclass{article}
\usepackage{lipsum}
\usepackage{atbegshi}
\usepackage{picture}
\usepackage{graphicx}
\newcommand*{\reflectthispage}{%
\ifodd\value{page}\edef\mtmargin{\oddsidemargin}\else\edef\mtmargin{\evensidemargin}\fi
\AtBeginShipoutNext{%
\AtBeginShipoutUpperLeft{%
\put(\dimexpr1in+\mtmargin\relax,-\dimexpr1in+\topmargin+\headheight+\headsep+\textheight+\footskip\relax){%
\reflectbox{\box\AtBeginShipoutBox}%
}}}}
\setbox\AtBeginShipoutBox=\vbox{\hspace{\mtmargin}\reflectbox{\box\AtBeginShipoutBox}}}}
\begin{document}
\section{Foo}
\lipsum[1-3]
\section{Bar}
\lipsum[1-3]
\section{Baz}
\reflectthispage
\lipsum[1-17]
\end{document}
\end{document}
