\setuppagenumbering[alternative=doublesided]

\starttext

    \definelayer
      [extrafiguretextlayer:l]
      [width=\overlaywidth,
       height=\overlayheight,
       corner={right,top},
       location={right,bottom},
       x=-1cm]

    \definelayer
      [extrafiguretextlayer:r]
      [width=\overlaywidth,
       height=\overlayheight,
       corner={left,top},
       location={left,bottom},
       x=-1cm]

    \startsetups extrafiguretext
        \signalouterrealign
        \setlayerframed
          [extrafiguretextlayer:\rightorleftpageaction rl]
          []
          [width=\marginwidth,
           align={lohi,middle},
           foregroundstyle=\tx]
          {\floatuserdataparameter{text}}
        \flushlayer
          [extrafiguretextlayer:\rightorleftpageaction rl]
    \stopsetups

    \defineoverlay
      [extrafiguretext]
      [\setups{extrafiguretext}]

    \setupfloat
      [figure]
      [background={foreground,extrafiguretext}]

    \dorecurse {20} {

        \startplacefigure[title=Dummy figure][text={margin dummy text #1}]
            \externalfigure[dummy][width=.8\textwidth]
        \stopplacefigure

    }

\stoptext
