Chapter, section, etc. numbers on left margin, aligned left

Here is a possibility, though it is very dirtily hacked:

\documentclass{memoir}
\usepackage[usehighlevels]{alnumsec}
  \alnumsectionlevels{1}{section,subsection,subsubsection,paragraph}
  \alnumsecstyle{aaaa}
  \surroundarabic[{.}][]{}{}
  \otherseparators{1}
  \alnumsecstyle{aaaa}
\usepackage{marginnote}
  \reversemarginpar
\usepackage{xpatch}

\chapterstyle{hangnum}

\makeatletter
\xpatchcmd\M@sect{\protected@edef}{\protected\def}{}{}
\xpatchcmd\@mn@@@marginnote{\@tempswatrue}{\@tempswafalse}{}{}
\marginparwidth 45\p@
\renewcommand{\printchapternum}{%
  \raisebox{5\p@}{\marginnote{\raggedright\chapnumfont\thechapter}}
}
\def\@seccntformat#1{\marginnote{\raggedright\csname the#1\endcsname}}
\makeatother

\begin{document}
\chapter{Some chapter}
\section{All these section numbers}
\subsection{should be aligned on the left}
\subsection{so that all the first digits}
\subsubsection{which is 1 in this case}
\paragraph{are underneath each other}
\end{document}