Voici une solution utilisant TikZ (pour ceux qui acceptent d'utiliser cette merveilleuse extension) :
\documentclass{article}
\usepackage{xcolor}
\usepackage[showframe,margin=2.5cm,top=2cm,bottom=2cm,a4paper]{geometry}
\usepackage{tikz}
\usetikzlibrary{backgrounds}
\makeatletter
\renewcommand\maketitle{%
\noindent\begin{tikzpicture}[remember picture,inner sep=0]
\node[text width=\linewidth,inner sep=0,line width=0](a){\@maketitle};
\begin{pgfonlayer}{background}
\begin{scope}[overlay]
\fill[fill=red!5]
\fill[fill=red!5] (current page.north west) west)
rectangle ([yshift=-.5em]a.south east -| current page.east);
\end{scope}
\end{pgfonlayer}
\end{tikzpicture}%
}%
\makeatother
\title{Titre}\author{Auteur}
\begin{document}
\maketitle
\section{Intro} Texte
\end{document}
