Apologies, but I forgot to paste my code. Here it is:
\documentclass{article}
\usepackage{luamplib}
\mplibtextextlabel{enable}
\mplibcodeinherit{enable}
\begin{document}
\begin{center}
\begin{mplibcode}
picture p;
beginfig(0);
draw (-36,0)--(36,0);
label.bot(textext("Hello"),origin);
p:=currentpicture;
endfig;
\end{mplibcode}
\end{center}
\begin{center}
\begin{mplibcode}
beginfig(1);
currentpicture:=p;
endfig;
\end{mplibcode}
\end{center}
\end{document}
