When installing from a hard drive partition, adding a mount point for it is
impossible. While not being able to modify the partition makes sense, you should
be able to add a mount point for it so the partition (e.g. /usr/local) is
mounted properly after boot.
*** Bug 159626 has been marked as a duplicate of this bug. ***
Committed, but have been unable to fully test due to lack of images. Looking at
this for later this week.