setbackgroundtemplate on title page within custom theme file

Question

I am working on a presentation theme for use with conference presentations. Most of our powerpoint themes use some type of background image on the first (title) slide, which I am trying to reproduce.

I can accomplish what I want by using

\usetheme{Mytheme}
\begin{document}
{
\usebackgroundtemplate{\includegraphics[width=\paperwidth]{MyBackground.jpg}}
\frame{\titlepage}
}
% Etc etc
\end{document}

This appears as I expect, with the image behind the presentation title, and only on the title slide.

What I want to do is move the use of the backgroundtemplate command in to the beamerthemeMytheme.sty file so that someone else using the theme down the road doesn't need to figure that particular command out. However, no amount of tweaking of setbeamertemplate{titlepage} { ... } has gotten this command to work.

I've seen examples using pgfimage to accomplish this effect, but my attempts have not come out nearly as cleanly as usebackgroundtemplate. For example, either:

\setbeamertemplate{title page}{
    \usebackgroundtemplate{\includegraphics[width=\paperwidth]{MyBackground.jpg}}
}

nor

{
    \usebackgroundtemplate{\includegraphics[width=\paperwidth]{MyBackground.jpg}}
    \setbeamertemplate{title page}{
    % ...
    }
}

have worked.

Is there a way to use usebackgroundtemplate (Or a similar command) from within a beamer style file?

Answer 1

Based on the fantastic answer https://tex.stackexchange.com/a/323638/36296 you could do the following little trick:

\documentclass{beamer}

\makeatletter
\def\ps@navigation@titlepage{%
  \usebackgroundtemplate{\includegraphics[width=\paperwidth]{example-image-duck}}% <-- define here the specific footline for the titlepage 
  \@nameuse{ps@navigation}}
\addtobeamertemplate{title page}{\thispagestyle{navigation@titlepage}}{}
\makeatother

\begin{document}

\begin{frame}
  \titlepage
\end{frame}

\begin{frame}
content...
\end{frame}

\end{document}