@samcarter \documentclass[10pt]{beamer}
\usetheme{Warsaw}
\usepackage{media9}
\usepackage{tikz}
\begin{document}
\begin{frame}[plain]
\begin{tikzpicture}[remember picture,overlay]
\node[anchor=south west, inner sep=0pt] at (current page.south west) {%
\includemedia[
addresource=a.mp4,
activate=pageopen,transparent,
flashvars={source=a.mp4},
width=\paperwidth,height=\paperheight
]{}{a.mp4}%
};
\end{tikzpicture}
\end{frame}
\end{document}