With `Umask 022` in mathopd.conf, editing pages works.
Hopefully this prevents someone else from spending ~2 hours figuring out why this wouldn't work. ;)
+
+> More generally, if your web server uses a nonstandard umask
+> or you're getting permissions related problems in the cgi log
+> when using ikiwiki, you can force ikiwiki to use a sane umask
+> via the `umask` setting in ikiwiki's own setup file. --[[Joey]]