After posting this closed question, I found a good solution for memoir on this page.
However, my margins settings are different then the ones he uses, and as a result, the widematter environment only works correctly on odd-numbered pages; on even-numbered pages, the widematter has the text run off to the wrong side, and it even goes off the page. Is there an easy way to debug this?
UPDATE:
Using @daleif's suggestion, I have the following MWE:
\documentclass[11pt,a4paper]{memoir}
\setstocksize{297mm}{210mm}
\settrimmedsize{\stockheight}{195mm}{*}
\settypeblocksize{671.6pt}{335.8pt}{*}
%\settypeblocksize{675pt}{337.5pt}{*}
%\settypeblocksize{*}{28pc}{2}
\setlrmargins{*}{*}{4}
\setulmargins{*}{*}{2}
\setmarginnotes{5mm}{45.23mm}{\onelineskip}
\setlength{\footskip}{3.0\baselineskip}
\setlength{\headwidth}{\dimexpr\textwidth+\marginparsep+1.0\marginparwidth\relax}
\checkandfixthelayout
\newlength{\extrawidth}
\setlength{\extrawidth}{\marginparsep}
\addtolength{\extrawidth}{\marginparwidth}
\marginparmargin{outer}
\strictpagechecktrue
\newenvironment{widematter}{
\begin{adjustwidth*}{0mm}{-\extrawidth}
\mpjustification}
{\end{adjustwidth*}}
\begin{document}
Here is some regular text that goes all the way to the margin and wraps around on the next line.
\begin{widematter}
Here is a test of the widematter environment, to see if the text is able to extend beyond the margin.
\end{widematter}
\end{document}
But this still doesn't work for even-numbered pages--the widematter text extends into the right margin, when it should be extending into the left margin...
(Also, is there any way to insert a page break into this MWE so that I can demonstrate the extra width on even-numbered pages?)
ANOTHER UPDATE:
Using Marco Daniel's fullwidth package, I have the following MWE:
\documentclass[11pt,a4paper]{memoir}
\setstocksize{297mm}{210mm}
\settrimmedsize{\stockheight}{195mm}{*}
\settypeblocksize{671.6pt}{335.8pt}{*}
%\settypeblocksize{675pt}{337.5pt}{*}
%\settypeblocksize{*}{28pc}{2}
\setlrmargins{*}{*}{4}
\setulmargins{*}{*}{2}
\setmarginnotes{5mm}{45.23mm}{\onelineskip}
\setlength{\footskip}{3.0\baselineskip}
\setlength{\headwidth}{\dimexpr\textwidth+\marginparsep+1.0\marginparwidth\relax}
\checkandfixthelayout
\usepackage[innermargin=-2.0cm,]{fullwidth}
\begin{document}
Testing some regular text that goes on past the first line, and then repeats.
Testing some regular text that goes on past the first line, and then repeats.
Testing some regular text that goes on past the first line, and then repeats.
\begin{fullwidth}[width=\linewidth+2cm]
Testing some regular text that goes on past the first line, and then repeats.
Testing some regular text that goes on past the first line, and then repeats.
Testing some regular text that goes on past the first line, and then repeats.
\end{fullwidth}
\end{document}
The first time it compiles, there are warnings about undefined references, but the fullwidth text extends past the margin on the right, which looks good. The second time it compiles, the warnings disappear, but now the text extends past the margin on the left (the wrong side)...
To clarify: for odd-numbered pages, the margin on the left represents necessary space for binding, whereas the margin on the right is just extra space (and visa-versa for even-numbered pages). With these settings, fullwidth is making the text extend into the binding margin, which is bad--I only want it to extend into the non-binding margin. I tried changing the code to have \usepackage[innermargin=0cm,]{fullwidth} and \begin{fullwidth}[width=\linewidth-2cm], but that only seems to work for odd-numbered pages--even-numbered pages still have fullwidth text go into the binding margin
adjustwidthdoesn't support pagebreaks. What's the problem with the linked packagefullwidth– Marco Daniel Dec 15 '11 at 01:31\setlength{\headwidth}{\textwidth+\marginparsep+1.0\marginparwidth}to\setlength{\headwidth}{\dimexpr\textwidth+\marginparsep+1.0\marginparwidth\relax}– Gonzalo Medina Dec 15 '11 at 17:15ANOTHER UPDATEsection in my question. – jamaicanworm Dec 15 '11 at 20:51\linewith+2cmand the innermargin is-2cm. I can't see any problems and warnings? – Marco Daniel Dec 15 '11 at 22:38fullwidthis making the text extend into the binding margin, which is bad--I only want it to extend into the non-binding margin. I tried changing the code to have\usepackage[innermargin=0cm,]{fullwidth}and\begin{fullwidth}[width=\linewidth-2cm], but that only seems to work for odd-numbered pages--even-numbered pages still havefullwidthtext go into the binding margin. – jamaicanworm Dec 15 '11 at 23:20\usepackage[outermargin=-2.0cm]{fullwidth}and then, in your document,\begin{fullwidth}[width=\linewidth+2cm]text text...\end{fullwidth}– Gonzalo Medina Dec 16 '11 at 02:09\setcounter{page}{2}in the preamble it still works fine. extending into the left margin. But if your problem is that the page break should happen inside the environment, then as I explained below,adjustwidth*does not support that. – daleif Dec 16 '11 at 16:08fullwidth, so that whenever I write\begin{fullwidth}LaTeX does\begin{minipage}{\linewidth} \begin{fullwidth}[width=\linewidth+4cm] \medskip, and whenever I write\end{fullwidth}LaTeX does\medskip \end{minipage} \end{fullwidth}? Thanks! – jamaicanworm Feb 18 '12 at 15:56