The convention for /usr is coincidental at best; it's definitely not true
   that /usr/X11R6 is shareable, and /usr/{local,pkg}/etc is somewhat annoying

Actually, /usr/X11R6 can be made shareable easily enough by changing
/usr/X11R6/lib/X11/xdm/xdm-config to point its errorLogFile and
pidFile to /var/log and /var/run (or whatever) as appropriate.  As far
as I can tell, these are the only files that get written into the X11
hierarchy by X itself, and shouldn't really be buried in the X11
filesystem anyway because of their machine-dependent nature.  I've run
with shared X11 trees using this approach and the systems work fine.

Perhaps it makes sense to change this in the X11 package we distribute
so that the X11 tree _would_ be sharable.

Cheers,
Brook