adding co-authors to customised title page for beamer presentation

I have customised my title page, using \title \date and \institute. I would like to add another line with co-authors, but I couldn't find any examples of how to do it. My ideas at the moment are

1. I could hi-jack \insertshortinstitute, since I don't use this in anywhere else.
2. I could define a new variable e.g. \def\mycoauthor{} (like on Use the values of \title, \author and \date on a custom title page) but then I don't know how to access it, especially since my maketitle definite is in a separate style file.

Here is a MWE showing how I create the title page at the moment, and an attempt to include coauthors:

\documentclass{beamer}
\usetheme{_MWE}

\title[short title]{TITLE TITLE TITLE TITLE TITLE TITLE}
\author[me]{A. U. Thor}
\def\coauthors{A. N. Other, J. Bloggs, J. Doe}
\institute[\coauthors]{My Institute}
\date{1.1.2012}

\begin{document}
\maketitle
\end{document}

beamertheme_MWE.sty:

\ProvidesPackage{beamertheme_MWE}

\makeatletter
\def\maketitle{%
\begin{frame}[plain]
{%
\vspace{10mm}
\parbox{122mm}%
{%
\centering
{\Large \inserttitle}\\
\vspace{8mm}
{\large \insertauthor}\\
\vspace{3mm}
{\large \insertshortinstitute}
\vspace{7mm}
{\small \insertinstitute}\\
\vspace{-5mm}
{\flushleft
\hspace{1mm}
LOGO1
\hspace{60mm}
\raisebox{0.5mm}{
LOGO2
\hspace{10mm}
}
}\\
\vspace{0mm}
{
\parbox{53mm}{\centering \small \insertdate\\
}
}
}
}
\end{frame}
}
\makeatother

incidentally... why doesn't the \centering command work for \insertshortinstitute, and why do I get an error "there's no line here to end" if I put a \\ after it, as I do with \author etc.?

-
I should add that I don't want to use the \thanks construct, since I don't think superscripts are the right way to refer to co-authors... unless someone knows a way to use \thanks and get the formatting I want? –  craq Oct 31 '12 at 13:19