'freebsd-update upgrade' hosed system - rescuable?

[cali clarke]

I just updated to 8.0-RELEASE with freebsd-update.

I ran:

# freebsd-update -r 8.0-RELEASE upgrade
<merge config files>
# freebsd-update install

I was told to reboot and run freebsd-update install again to complete the
process.

I rebooted into single user mode, mounted /usr and /var and somehow forgot
to
mount / as read-write.

Of course, freebsd-update didn't check to see if it could actually write to
all the
directories it needed to write to first and catastrophically wrote half an
upgrade
(which it now appears to have deleted the "source" files for) and has broken
my
system (most of the stuff in /usr/bin now depends on libraries in /lib that
were
never written).

What's the quickest and cleanest way out of this mess?

I'll submit a PR if my system ever comes back up.

Please CC, i'm not subscribed.