This page outlines the steps that would be required for each of the temporary directory scenarios in the mailing list thread (http://www.pmichaud.com/pipermail/pmwiki-users/2007-February/038723.html).
Installing pmwiki (if safe_mode off):
chmod 2777 pmwiki_root
Installing pmwiki (safe_mode on, or site cannot do "chmod 2777"):
mkdir pmwiki_root/wiki.d
chmod 777 pmwiki_root/wiki.d
mkdir pmwiki_root/tmp
chmod 777 pmwiki_root/tmp
For now we'll assume the top-level writable directory is called data.d/ (likely to change).
Installing pmwiki, safe_mode off:
chmod 2777 pmwiki_root/data.d
or chmod 777 pmwiki_root/data.d
Installing pmwiki, safe_mode on:
mkdir pmwiki_root/data.d/pages
chmod 777 pmwiki_root/data.d/pages
mkdir pmwiki_root/data.d/tmp
chmod 777 pmwiki_root/data.d/tmp
Presumably the data.d/ directory could be included in the distribution as an empty directory. One potential downside to this would occur for Mac OS/X systems. If a Mac OS/X system attempts to do a PmWiki upgrade by dragging-and-dropping a new pmwiki root on top of an existing pmwiki root, and the distribution contains an (empty) data.d/ directory, that empty directory would override any existing data.d/ directory, thus destroying any pages that happened to exist.
This page may have a more recent version on pmwiki.org: PmWiki:TempDirNotes, and a talk page: PmWiki:TempDirNotes-Talk.