Use page number in longtable

Question

I am using longtable and want to access the page number from within the longtable, as in this bare-bone example:

\documentclass[a4paper]{article}
\usepackage{geometry}
\usepackage{longtable}

\begin{document}
\begin{longtable}{l}

longtable header: this is page \thepage
\endhead

A \\ 
A \\ 
A \\ 
... however many rows ...
... needed to exceed one page ...

\end{longtable}
\end{document}

However, the generated PDF always shows this is page 1 in the longtable header, even if it is on page 2.

According to a comment to this question, longtable "saves the head in a box and doesn't re-set it each page". The comment seems to suggest that the rolling page number simply cannot be accessed from within a longtable without a major rewrite. Do I understand that correctly? (I certainly hope that I do not understand it correctly, so there's still some hope that it can be done.)

Answer 1

This sort of works. I can't figure out where the left margin comes from though. Also, for some reason the page counter wasn't working.

\documentclass[a4paper]{article}
\usepackage{geometry}
\usepackage{longtable}
\usepackage{everypage}

\newcounter{mypage}
\setcounter{mypage}{1}

\makeatletter
\AddEverypageHook{\stepcounter{mypage}%
  \ifdim\ht\LT@head>0pt
    \dimen0=\ht\LT@head
    \global\setbox\LT@head=\vbox to\dimen0{\strut
      longtable header: this is page \themypage}%
  \fi
}
\makeatother

\begin{document}
\begin{longtable}{l}
longtable header: this is page \thepage
\endhead
\rule{0.7\textwidth}{0.7\textheight}\\
\rule{0.7\textwidth}{0.7\textheight}\\
\rule{0.7\textwidth}{0.7\textheight}
\end{longtable}
\end{document}