Comment 4 for bug 583542

This may be out of scope for a bug report, but why not change the way an upstart job describes its start conditions? ssh, for example, could supply a script which checks if /usr is mounted. The script(s) can be run after every upstart job completes, and when all conditions are met the new jobs are started.

In the meantime, I'll check out the nobootwait workaround.