\documentclass{article}

\usepackage{geometry}
\usepackage{pdfpages}

\usepackage{amsmath,amssymb,amsthm}
\usepackage{commath,mathtools}
\usepackage{parskip}
\usepackage{graphicx}
\usepackage{xcolor}
\usepackage[dutch]{babel}
\usepackage[bookmarksnumbered]{hyperref}
\usepackage{highlightlatex}

\usepackage{fancyhdr}

\geometry{
    a6paper,
    %landscape,
    margin=2cm,
    left=1cm,
    right=1cm,
    paperheight=12cm
}

\pagestyle{fancy}

\updatehighlight{
    name=default,
    add={
        \fancyhead, \fancyfoot, \geometry
    }
}

\fancyhead[L]{\hll|\\fancyhead[L]\{\}|}
\fancyhead[C]{\hll|\\fancyhead[C]\{\}|}
\fancyhead[R]{\hll|\\fancyhead[R]\{\}|}

\fancyfoot[L]{\hll|\\fancyfoot[L]\{\}|}
\fancyfoot[C]{\hll|\\fancyfoot[C]\{\}|}
\fancyfoot[R]{\hll|\\fancyfoot[R]\{\}|}

\begin{document}
    \section{Lorem ipsum}

    Lorem ipsum dolor sit amet, consectetur adipiscing elit. Suspendisse
    tincidunt eleifend enim, ut pharetra mi.
    \medskip

    \begin{hllblock}[gobble=8,linewidth=\textwidth,
		framexleftmargin=0.25em,xleftmargin=0.25em]
        \usepackage{geometry}
        \usepackage{fancyhdr}

        \geometry{
            a6paper,
            %landscape,
            margin=2cm,
            left=1cm,
            right=1cm,
            paperheight=12cm
        }

        \pagestyle{fancy}
        \fancyhead[L]{Linksboven!}
        \begin{document}
            ...
        \end{document}
    \end{hllblock}
\end{document}