\documentclass{article}

\usepackage{amsmath,environ}

\NewEnviron{myEqn}[1]{%
  \begin{flalign}
    #1 && \BODY &&
  \end{flalign}
}

\begin{document}

\newcommand{\mycommand}{%
  \text{mycom}% ...or a complicated macro
}

\begin{myEqn}{\mycommand}
  1 + 1 = 2
\end{myEqn}

\end{document}