tcolorbox poster - draw on the background behind the foreground

Question

Below is a small tcolorbox poster example. Does anyone know how to draw on top of the background image, but behind the posterboxes.

As a subquestion, the tcolorbox manual, page 150, about interior style image mentions that one can combine fill stretch image and interior style to blend picture and background. Hoever it is not clear to me how one would add suitable paths for this.

\documentclass[a5paper]{memoir}
\usepackage[poster]{tcolorbox}
\begin{document}

\begin{tcbposter}[
    coverage = {
      spread,
      interior style = {
        fill stretch image=example-image-c.pdf,% from mwe bundle
      },
      top=2em,
      bottom=2em,
      left=2em,
      right=2em,
    },
    poster = {
      % showframe, 
      columns=3,
      rows=3,
      spacing=2em,
    },
    boxes = {},
    fontsize = 16pt,        
]

\posterbox{name=title,column=1,below=top,span=3}{
  Test
}

\end{tcbposter}
\end{document}

Answer 1

To answer my own question here is a method to draw on top of the background, behind the foreground. Just remember to place relative to the interior node, everything outside this node is clipped away.

\documentclass[a5paper]{memoir}
\usepackage[poster]{tcolorbox}
\begin{document}

\begin{tcbposter}[
    coverage = {
      spread,
      interior style = {
        fill stretch image=example-image-c.pdf,% from mwe bundle
      },
      overlay={% here
          \fill[red] (interior.north) circle (3cm);
      }, 
      top=2em,
      bottom=2em,
      left=2em,
      right=2em,
    },
    poster = {
      % showframe, 
      columns=3,
      rows=3,
      spacing=2em,
    },
    boxes = {},
    fontsize = 16pt,        
]

\posterbox{name=title,column=1,below=top,span=3}{
  Test
}

\end{tcbposter}
\end{document}