documentclass{memoir}
  usepackage{letltxmacro}
  LetLtxMacro{oldsection}{section}
  renewcommand{section}[2][]{oldsection[#1]{#2}index{#1}label{sec:#1}}

  LetLtxMacro{oldchapter}{chapter}
  renewcommand{chapter}[2][]{oldchapter[#1]{#2}index{#1}label{chap:#1}}

begin{document}
  chapter[ABC]{ABC}
    section[One]{One}
    section{Two}
    section*[Three]{Three}
     Test reference ref{chap:ABC}
     Test reference ref{chap:XYZ}
     Test reference ref{sec:One}
     Test reference ref{sec:Two}
   chapter{DEF}
   chapter*[XYZ]{XYZ}
end{document}