I see that splitjon can already split between single-line and multi-line LaTeX environments as follows:
\begin{itemize} \item Hello \item Goodbye \end{itemize}
\begin{itemize}
\item Hello
\item Goodbye
\end{itemize}
I would find it useful if splitjoin also worked with commands. For example:
\abstract{Words words words foo bar baz.}
\abstract{%
Words words words foo bar baz.
}
Maybe I'll give this a shot myself in a week or so.