Center longtable horizontally over two pages in landscape

Question

I have a longtable in a landscape environment that ranges over two pages, and where I have introduced a page break where it is most appropriate contentwise. The table including descriptions is too long to be displayed on one page only but doesn't really fill two full pages either. I therefore split it in half, which gives me a leftward alignment. This doesn't look too appealing, so I'm trying to center it horizontally (i.e., when viewed in portrait mode). For the first page, I have succeeded in doing that, but not for the second. Here's my code:

\begin{landscape}
  \begin{centering}
    \begin{longtable}{lcccccccc}
      \noalign{\vspace*{\fill}} % This is to center "from the left-hand side" (from a portrait POV)
      \caption{My caption}\\
      \label{MYLABEL}
      ...
      \multicolumn{9}{c}{(continued on next page)}\\
      \noalign{\vspace*{\fill}} % This is to center "from the right-hand side" (from a portrait POV)
      \pagebreak % Manual page break so it "looks better"
      \noalign{\vspace*{\fill}} % This is to center "from the left-hand side" on the second page, which doesn't work.
      \caption*{My caption (continued)}\\
      ...
    \end{longtable}
  \end{centering}
  \noindent {\scriptsize \textit{Notes} --- Here's a more detailed description of the table.
  }
\end{landscape}

As already stated in the comments \noalign{\vspace*{\fill}} doesn't change anything on the second page, whereas the vspacing works just fine on the first page. What am I doing wrong/missing?

Answer 1

longtable boxes the last page (which stops your glue stretching). It probably shouldn't, this removes the offending box.

\documentclass{article}
\usepackage{lscape,longtable}

\makeatletter
\let\zzLT@output\LT@output
\def\unboxlastpage#1\@colht\vbox{#1\@colht}
\def\LT@output{\expandafter\unboxlastpage\zzLT@output}
\makeatother
\begin{document}


\begin{landscape}
  \begin{centering}
    \begin{longtable}{lcccccccc}
      \noalign{\vspace*{\fill}} % This is to center "from the left-hand side" (from a portrait POV)
      \caption{My caption\label{MYLABEL}}\\
      \multicolumn{9}{c}{(continued on next page)}\\
      \noalign{\vspace*{\fill}} % This is to center "from the right-hand side" (from a portrait POV)
      \pagebreak % Manual page break so it "looks better"
      \noalign{\vspace*{\fill}} % This is to center "from the left-hand side" on the second page, which doesn't work.
      \caption*{My caption (continued)}\\
      ...\\
 \noalign{\vspace*{\fill}}
    \end{longtable}
  \end{centering}
  \noindent {\scriptsize \textit{Notes} --- Here's a more detailed description of the table.
  }
\end{landscape}

\end{document}