\documentclass{article}
\usepackage{fancyhdr}
\makeatletter
\newcommand{\default@chead}{My default}
\let\mrmrunningtitle\chead
\mrmrunningtitle{\default@chead} % put this in your class
\makeatother
\mrmrunningtitle{User defined title} % user uses the same thing
\pagestyle{fancy}
\begin{document}
Some text.
\end{document}