%\setuppapersize[A5][A5]

\startuniqueMPgraphic{wave}{wl}
numeric periods, width;
path wave;
periods := 3 + 1/4;
width   := periods * 2*pi;
wl      := \MPvar{wl};
ll      := OverlayWidth-wl;
wave    := (origin for x=0 step 0.1 until width:
             .. (x,x*sin(x))
            endfor) xscaled -1; % mirror (no shift correction needed)
pickup pencircle scaled 1pt;
draw reverse(wave xscaled (wl/width) yscaled (OverlayHeight/2width))--(ll,0);
\stopuniqueMPgraphic

\definelayer[decorations][position=yes,option=test,location=r,dx=-\cutspace]
\setupbackgrounds[page][background=decorations]
\defineoverlay[wave][\uniqueMPgraphic{wave}{wl=\cutspace}]


\starttext
\showframe[text]

{\ss\bfc A chapter title}
\blank[small]
\framed[frame=off,width=broad]
 {\ss\bfx \date \hfill Author Name}%
\setlayer[decorations]{\framed[background=wave,frame=off,width=\dimexpr\cutspace+\makeupwidth,height=2\lineheight,offset=overlay]{}}

\input knuth

\stoptext