Formula label outside of centred formula

Question

This is intentionally a duplicate of Formula label outside of formula . The accepted answer to the linked question does not appear to give the requested behaviour, as this MWE demonstrates:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\subsection{Un-numbered equations work}

\noindent Centred equation using \texttt{align*}:
\begin{align*}
    a &= b + c + d
\end{align*}
Centred equation using \texttt{flalign*} and \texttt{phantom}
\begin{flalign*}
    \text{(Very very very long label)}
    &&
    a &= b + c + d
    &
    \phantom{\text{(Very very very long label)}}
\end{flalign*}

\subsection{Numbered equations don't work}

\noindent Centred equation using \texttt{align}:
\begin{align}
    a &= b + c + d
\end{align}
``Centred'' equation using \texttt{flalign} and \texttt{phantom}
\begin{flalign}
    \text{(Very very very long label)}
    &&
    a &= b + c + d
    &
    \phantom{\text{(Very very very long label)}}
\end{flalign}
Removing \texttt{phantom} makes things worse:
\begin{flalign}
    \text{(Very very very long label)}
    &&
    a &= b + c + d
    &
\end{flalign}

\end{document}

Output with markup:

It appears that, for a numbered equation, the "centred" part of the equation is not centred between the margins, but rather centred in the space between the left margin and the left-hand side of the equation number.

Is it possible to get the "centred" part of the equation centred between the margins?

Edit: I'm not set on using flalign; other solutions are fine. I just used flalign in the MWE because the objective of the MWE is to justify the new question by showing why the previous answer (which uses flalign) doesn't work

Answer 1

What you have observed is the intended behavior of flalign* and flalign:

User’s Guide for the amsmath Package, Page 8: The environment flalign (“full length alignment”) stretches the space between the equation columns to the maximum possible width, leaving only enough space at the margin for the equation number, if present.

To achieve your description, \usetagform from the mathtools package comes to mind:

\documentclass{article}

%\usepackage{amsmath}
\usepackage{mathtools}% loads `amsmath'
\newtagform{zerowidth}{\llap\bgroup(}{)\egroup}

\begin{document}

\subsection{Numbered equations}

\noindent Centred equation using \texttt{align}:
\begin{align}
    a &= b + c + d
\end{align}
``Centred'' equation using \texttt{flalign} with user-defined tag style \texttt{zerowidth}:
\usetagform{zerowidth}
\begin{flalign}
    \text{(Very very very long label)}
    &&
    a &= b + c + d
    &
    \phantom{\text{(Very very very long label)}}
\end{flalign}
\usetagform{default}% don't forget to switch back

\end{document}