How can I make a standalone document loading siunitx work?

Question

I've got a standalone document that uses siunitx:

% subdocument.tex
\documentclass{standalone}
\usepackage{siunitx}
\begin{document}
\SI{10}{\kilo\newton}
\end{document}

I'd like to include this standalone document as a sub-document in a different document, using the standalone package. Because I don't want to manually keep track of which packages are needed for the sub-document, I want to use the subpreambles=true option of standalone:

% main.tex
\documentclass{article}
\usepackage[subpreambles=true]{standalone}

\begin{document}
\input{subdocument.tex}
\end{document}

However, if I do that, I get an error message:

! Undefined control sequence.
<argument> \l__siunitx_option_deactivate_clist

I could fix this by loading siunitx in main.tex, but I hope that there is a different way around this.

---

Here's a MWE:

\documentclass{article}
\begin{filecontents}{subdocument.tex}
    \documentclass{standalone}
    \usepackage{siunitx}
    \begin{document}
    \SI{10}{\kilo\newton}
    \end{document}
\end{filecontents}

% This works (loading siunitx only in the main document)
%\usepackage[subpreambles=false]{standalone}
%\usepackage{siunitx}

% This doesn't (loading siunitx in the sub-document), but it would be nice if it did:
\usepackage[subpreambles=true]{standalone}

\begin{document}
\input{subdocument.tex}
\end{document}

Answer 1

It turns out that just adding \AtBeginDocument{\renewcommand{\AtBeginDocument}[1]{}} before loading the standalone package makes the MWE compile correctly. This is definitely not the correct solution since this throws away all the things that the siunitx package thinks need to be done in \AtBeginDocument. If instead we go with an MWE of

\documentclass{article}
\usepackage{etoolbox}
\newcommand{\AtBeginSubDocument}{}
\AtBeginDocument{\renewcommand{\AtBeginDocument}[1]{\apptocmd{\AtBeginSubDocument}{#1}{}{}}}
\usepackage[subpreambles=true]{standalone}

\AtBeginDocument{\meaning\AtBeginSubDocument}
\AtBeginDocument{\AtBeginSubDocument}
\begin{document}
\input{subdocument.tex}
\end{document}

The MWE compiles and shows that about 2 pages worth of content is being checked in \AtBeginDocument. Much of this looks like checks to see if packages are loaded (i.e., @ifpackageloaded) and there may be package combinations that cause problems.