Ignoring specific ampersand formatting on a specific line of an align block

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{align}
&& \llap{$\displaystyle \sum_0^7 x-(1+2+3+4+5+6+7)$} &= 0\\
      a& = b &                     b &= c\\
      c& = d  &                    d &= e
\end{align}

\end{document}

Use \span to merge cells. You need one \span per & you want to ignore. For example, your sample output could be produced like this:

\begin{align*}
  \text{really really really long equation} \span \span &= a \\ 
  a &= b & b &= c \\
  c &= d & d &= e
\end{align*}