\documentclass[listchar]{sciencetown}

\makeatletter
\catcode`\:11\relax

\long\def\:gap#1#2{%
  \expandafter\xdef\expandafter#1\expandafter{%
    \expandafter\unexpanded\expandafter{#1#2}}}

\begin{document}

\EVERY{Item}{
  \endgroup
  \expandafter\:gap\csname \:id :l\endcsname{\let\@placed\relax}
  \begingroup
}

\MAP{Item}{
  \endgroup
  \expandafter\:gap\csname \:id :l\endcsname{\def\@placed{}}
  \begingroup
}

\EVERY{Place}{\MYitems}
\EVERY{PC}{\MYitems}

\MAP{Item}{
  \ifx\@placed\relax
  \typeout{! \expandafter\noexpand\MYCOMMAND is referenced in 
    \expandafter\noexpand\@referrer, but not placed}
  \fi
}

\EVERY{Notebook}{
  \def\startnotebook{}
  \def\endnotebook{}
  \newenvironment{page}{}{}
  \def\nbref{}
  \let\@referrer\MYCOMMAND
  \MYtext
}
\EVERY{Green}{
  \let\@referrer\MYCOMMAND
  \MYtext
}
\EVERY{Abil}{
  \let\@referrer\MYCOMMAND
  \MYtext\MYeffect
}
\end{document}