captionof in longtable gives Misplaced \noalign

longtable has its own definition of \caption which makes using \captionof doubly difficult: Even if you circumvent the Misplaced \noalign, you'll notice that the floattype table is hardcoded...

As \captionof will always call \caption it's a bit hard to patch it in a way which will work inside and outside of longtable without completely rewriting longtables version of \caption. Here's an idea based on \multicolumn:

\documentclass{article}
\usepackage{longtable}
\usepackage{etoolbox}

\makeatletter
\let\o@caption\caption
\newcommand\LT@captionof[2]
{%
  \multicolumn{\LT@cols}{p{\textwidth}}{\def\@captype{#1}\o@caption{#2}}%
}
\patchcmd\LT@array{\let\caption\LT@caption}
{\let\caption\LT@caption\let\captionof\LT@captionof}{}{}
\makeatother

\begin{document}

\listoffigures

\begin{longtable}{cc}
    \captionof{figure}{This gives a Misplaced noalign error \label{fig:dummy1}} \\
     %\caption[This]{Works} \\
    Test & test 2 \\
    Test & test 2 \\
% later need to include this: \addtocounter{table}{-1} %
\end{longtable}

reference to figure \ref{fig:dummy1}.

\end{document}