zref + mplib: a proof of concept

mpzref.tex

\documentclass[a4paper]{article}
\usepackage{luamplib, zref-savepos, atbegshi, picture}
\begin{document}
\AtBeginShipoutNext{\AtBeginShipoutUpperLeft{%
    \put(0,-\paperheight){%
      \begin{mplibcode}
        beginfig(0);
        x0 := \mpdim{\zposx{z0x} sp};
        y0 := \mpdim{\zposy{z0y} sp};
        x1 := \mpdim{\zposx{z1x} sp};
        y1 := \mpdim{\zposy{z1y} sp};
        drawarrow z0{dir 45} .. {dir 45}z1 withcolor red;
        setbounds currentpicture to
        origin--(0,\mpdim{\paperwidth})--
        (\mpdim{\paperwidth},\mpdim{\paperheight})--
        (\mpdim{\paperheight},0)--cycle;
        endfig;
\end{mplibcode}}}}
\fbox{This}\zsavepos{z0x}\vadjust pre{\zsavepos{z0y}} is a test line.
This is a test line.
This is a test line.
This is a test line.
This is a test line.
This is a test line.
This is a test line.
This is a test line.
This is a test line.
This is a test line.
This is a test line.
This is a test \zsavepos{z1x}\vadjust{\zsavepos{z1y}}\fbox{line}.
\end{document}