Aligning All Equations in a Document at Equality Sign

I only considered implementing a global alignment as egreg hinted in comments that it was a bad idea. Alignment is supposed to highlight the corresponding parts of a display and that meaning is destroyed if you align everything. But...

\documentclass{article}

\usepackage[fleqn]{amsmath}
\usepackage{etoolbox}
\setlength\mathindent{3cm}
\makeatletter

\def\align@preamble{%
   &\hfil
    \strut@
    \setboxz@h{\@lign$\m@th\displaystyle{##}$}%
    \ifmeasuring@\savefieldlength@\fi
    \llap{\set@field}%
    \tabskip\z@skip
   &\setboxz@h{\@lign$\m@th\displaystyle{{}##}$}%
    \ifmeasuring@\savefieldlength@\fi
    \rlap{\set@field}
    \hfil
    \tabskip\alignsep@
}


\def\a{One two three four five six seven. }
\def\b{\a\a\a Red yellow blue green. \a\a\a\a}
\begin{document}

\b\b\a
\begin{align}
  1+2+3+4 &=4\\
x+y &=z
\end{align}

\b\b\b

\a\a\b

\a
\begin{align}
  1&=a+y+b+w+x+n+m+s+d+e\\
  2&=4
\end{align}

\b
\begin{align}
  1&=0\\
2&=9
\end{align}
\end{document}

the macros here are not particularly general or robust but show a basic idea of using a flush left but very indented setting along with hiding the width of the first column.

Equations can be left aligned by option fleqn. The left indentation can be controlled by register \mathindent (package amsmath). Then the left part of the equation can be written via \llap in the indentation space:

\documentclass[fleqn]{article}
\usepackage{amsmath}
\setlength{\mathindent}{3em}
\newcommand*{\ml}[1]{%
  \llap{$\displaystyle#1$}%
}
\begin{document}
\begin{gather*}
  \ml{a} = b + c + d\\
  {} + e + f + g
\intertext{A small amount of text can go htere with $x=2$ inline math,
but not a lot of text.}
  \ml{k} = l + m + n + m + n + m + n\\
  {} + o + p + q\\
  \ml{r + s} = t + u
\end{gather*}
\end{document}