[O] Beamer export: metropolis-theme specific question

[zang_news]

Dear fellow Orgers,

I like the metropolis beamer theme a lot [1], and I use it with Org 
frequently. metropolis has a \plain macro that inserts a new, minimally 
styled slide optimal for a short statement or an image. In plain latex, 
I would do e.g.

\begin{frame}
\frametitle{a normal frame with title etc.}
some content
\end{frame}

\plain{some text on a minimal frame}

\begin{frame}
\frametitle{another normal frame with title etc.}
some other content
\end{frame}

I just cannot get my head around how to tell Org to do this! If I do

* a normal frame with title etc.
some content

#+BEGIN_LATEX
\plain{another normal frame with title etc.}
#+END_LATEX

* another normal frame with title etc.
some other content

this of course does not work. Has anyone come across this as well and is 
willing to share a solution?

Thanks!
Christian

[1] https://github.com/matze/mtheme/