Rule in textarea every n points

The following solution is independent from the output routine and uses a eso-pic like approach:

\documentclass[11pt]{article}
\usepackage{lipsum}
% \usepackage[showframe]{geometry}

\usepackage{atbegshi,picture}
\makeatletter
\AtBeginShipout{%
  \AtBeginShipoutUpperLeft{%
    \put(\dimexpr
      \ifodd\value{page}\oddsidemargin\else\evensidemargin\fi + 1in\relax,
      -\dimexpr\topmargin + 1in + \headheight + \headsep\relax){%
      \begin{picture}(0,0)%
        \setlength{\unitlength}{13bp}%
        \setlength{\dimen0 }{0pt}%
        \@whiledim\dimen0<\dimexpr\textheight+1sp\relax\do{%
          \put(0,-\dimen0){\line(1,0){\textwidth}}%
          \addtolength{\dimen0 }{\unitlength}%
        }%
      \end{picture}%
    }%
  }%
}
\makeatother

\makeatletter
\setlength{\topskip}{13bp}
\renewcommand\normalsize{%
   % \baselineskip to 13bp for \normalsize
   \@setfontsize\normalsize\@xipt{13bp}%
   % the following values are taken from size11.clo.
   \abovedisplayskip 11\p@ \@plus3\p@ \@minus6\p@
   \abovedisplayshortskip \z@ \@plus3\p@
   \belowdisplayshortskip 6.5\p@ \@plus3.5\p@ \@minus3\p@
   \belowdisplayskip \abovedisplayskip
   \let\@listi\@listI
}
% \parskip without glue:
% \setlength{\parskip}{0pt}
% or with positive \parskip:
\usepackage{parskip}
\setlength{\parskip}{13bp}
\makeatother

\begin{document}
\lipsum[1-3]
\end{document}

Something like this (I also changed baselineskip and parskip to the grid)

\documentclass{article}

\makeatletter
\renewcommand\normalsize{%
   \@setfontsize\normalsize\@xpt{13bp}
   \abovedisplayskip 10\p@ \@plus2\p@ \@minus5\p@
   \abovedisplayshortskip \z@ \@plus3\p@
   \belowdisplayshortskip 6\p@ \@plus3\p@ \@minus3\p@
   \belowdisplayskip \abovedisplayskip
   \let\@listi\@listI}
\normalsize
\setlength\parskip{13bp}

\def\@oddhead{%
\setlength\unitlength{13bp}
\raisebox{-\topskip}{\begin{picture}(0,0)%
\multiput(0,0)(0,-1){40}{\line(1,0){30}}%
\end{picture}}\hfill}
\makeatother

\def\a{One two three four five. }
\def\b{\a\a Red green blue. \a\a}
\def\c{\b\b Black white. \b\a\b}


\begin{document}

\c

\c

\c

\end{document}

That are simple lines, can also be done with \rule, package pict2e or something similiar.

\documentclass{article}
\usepackage{pstricks,multido}

\begin{document}
\setlength\baselineskip{13bp}\noindent%
\multido{\iA=0+13}{45}{\psline[linecolor=gray](0,-\iA bp)(\linewidth,-\iA bp)}%
%
foo\\
bar\\
baz\\
foobar\\
foobarbaz

\end{document}

and the same with \rule

\documentclass{article}
\usepackage{multido}
\begin{document}
\makeatletter%
\newdimen\tempL%
\setlength\baselineskip{13bp}\noindent%
\makebox(0,0)[t]{\multido{\iA=0+13}{40}{%
\put(0,-\strip@pt\tempL){\rule{\linewidth}{0.5pt}}%
\advance\tempL by 13bp}}
\makeatother%
%
foo\\
bar\\
baz\\
foobar\\
foobarbaz

\end{document}