Some time ago I was wondered finding empty '/$' directory in the root of filesystem. Debugging showed, that it was created by openrc (for now =sys-apps/openrc-0.24.2), starting a buggy custom init script. According to FHS, ordinar users should not be able to create/modify files outside /home/${user}, /var and /tmp (root also can edit configs in /etc). All other files in /usr, /lib and other system directories must be handled with package menagement system (i.e. sys-apps/portage). For a years ago sys-apps/portage switched from direct invocation of mkdir and other core utilities to usage built-in functions (dodir and others). Probably, openrc should follow ebuild tool, providing safe built-ins for all necessary functions (allowing filesystem write only for creation pid files into /var/run, empty log files into /var/log for services, configured for file logging, creation file and writing its own log and, possibly some dynamic mount points somewhere in /tmp) and at least warning, when init script uses direct invocation of core utilities. P.S. Bug #438964 describes similiar issue, possibly valid exception.
I understand your concern; however, I do not see it as something OpenRC can fix for several reasons. - OpenRC doesn't have a way of knowing where a service needs write access. - Most of the "basic utilities" you are referring to are not part of OpenRC, so I think they would all have to be updated to respect any scheme that does this. To give you a short answer, I do not see a clean way this could be done internally to OpenRC. Patches, however, are definitely welcome, so if you can come up with an implementation let me know. Thanks, William