How customized can an mfsroot be?

[Peter Steele]

>If FreeBSD cannot write to /tmp or /var on boot, it automatically 
>creates a MFS filesystems for those mountpoints and mounts them during boot.  You don't need to do anything.
>
>It works as the same readonly compactflash environments out there.

What incidentally does /var get populated with? Our image has a custom directory under /var but this did not show up in the MFS versions of this directory. I can get around this but I wonder what else might not be included?