You won't (or at least, should not be able to). / is mounted read-only
by design. If you want files to appear there, you have to build an
image from source and include them in the build process.
The only parts of the filesystem that are writable at runtime are
/home, /var, and /mnt/stateful_partition (which contains /home and
/var).
That doesn't mean you cannot loop mount
/mnt/stateful_partition/something elsewhere in the filesystem, but you
won't be able to do that on /, /usr, or /opt, of course (because they
have rather important stuff in them).
Anyone figure out how to get root to mount RW?
Cute. Design docs I read some time back seemed to imply that readonly
root partition was going to be kernel-enforced. It's interesting to
see that it isn't.
Note that there are other caveats, not the least of which is that your
changes could get "blown away" by an auto-update when the system
reboots into the alternate root filesystem (there are two, A-ROOT and
B-ROOT). The updater does run in dev mode, but it is supposed to
update signed images if running under the official Chrome OS builds. I
don't know how the updater will behave if the root fs is modified
(breaking the signature on the active root filesystem).