mplib figure as a page background

mpbackground.tex

%\pdfcompresslevel=0
%\pdfobjcompresslevel=0
\input luamplib.sty
\mplibsetformat{metafun}
\newbox\MpPageBg
\setbox\MpPageBg\hbox{%
  \mplibcode
  beginfig(0);
  draw btex D R A F T etex scaled 14 rotated 60
    withcolor transparent("normal", 0.2, black);
  endfig;
  \endmplibcode
}\wd\MpPageBg=0pt\ht\MpPageBg=0pt\dp\MpPageBg=0pt
\edef\pagebody{\unexpanded\expandafter{\pagebody\copy\MpPageBg}}
\count255=0 \loop
  This is a test line. This is a test line.
  This is a test line. This is a test line. \endgraf
  \ifnum\count255<120 \advance\count255 by 1 \repeat
\bye