You don't need fancyhdr for this. For example, you can use eso-pic to place content on the page arbitrarily. Below I've built an interface that places content on the left side of the page as needed:

\documentclass{report}
\usepackage{fancyhdr,lipsum,graphicx}
\usepackage{eso-pic}
\fancypagestyle{sidelabel}{% Not really needed
\renewcommand{\headrulewidth}{0.4pt}
\fancyfoot[C]{--\thepage--}
}
\providecommand{\sidelabeltext}{}
\AddToShipoutPictureFG{% Place this content on the ForeGround of every page
\AtTextCenter{%
\hspace*{\dimexpr-.5\textwidth-\sidelabelsep}%
\rotatebox{90}{%
\makebox[0pt]{%
\sidelabeltext%
}%
}%
}%
}
\newlength{\sidelabelsep}
\setlength{\sidelabelsep}{\marginparsep}
\newcommand{\setsidelabeltext}[1]{\renewcommand{\sidelabeltext}{#1}}
\newcommand{\setsidelabelsep}[1]{\setlength{\sidelabelsep}{#1}}
\pagestyle{sidelabel}
\begin{document}
\setsidelabeltext{This is some side text}
\lipsum[1-6]
\setsidelabeltext{This is some very very very very very very very very very very very
very very very very very very very very very very very very very long side text}%
\setsidelabelsep{0.5in}
\lipsum[1-6]
\setsidelabeltext{}
\lipsum[1-6]
\setsidelabeltext{This is some side text}%
\setsidelabelsep{20pt}
\lipsum[1-6]
\end{document}
\setsidelabeltext{<stuff>} updates what is printed on the left of the page, while \setsidelabelsep{<len>} updates the length that <stuff> is printed from the text. Default is \marginparsep.