\gdef\mpxshipout{\shipout\hbox\bgroup%
\setbox0=\hbox\bgroup}%
\gdef\stopmpxshipout{\egroup \dimen0=\ht0 \advance\dimen0\dp0
\dimen1=\ht0 \dimen2=\dp0
\setbox0=\hbox\bgroup
\box0
\ifnum\dimen0>0 \vrule width1sp height\dimen1 depth\dimen2
\else \vrule width1sp height1sp depth0sp\relax
\fi\egroup
\ht0=0pt \dp0=0pt \box0 \egroup}
\mpxshipout% line 1 ./mptextmp.mp
Kurve 1%
\stopmpxshipout
\end{document}