vonbraun, runlevels mean something very different than they used to, so I don't believe runlevel 1 is actually where you want to be.
Sine you want to run ASAP, there are two options.
If you can work without writing to the root filesystem, and your data is guaranteed to be on the root filesystem, you can use the mounting event.
start on mounting MOUNTPOINT=/
The mounting event is a "hook", so mountall will wait for anything that uses it to unblock. You definitely must use the 'task' keyword so that your checks can run fully before mountall is unblocked.
At this point in the boot, the root filesystem is still readonly, but should be available. Other filesystems may not be available, so be aware (/var/run, in particular, can be tricky in this way). Do not use "AND" with another mounting event as that will lock up boot.
If you need to write to the root FS, you can use
start on mounted MOUNTPOINT=/
This will block mountall again, but after / is remounted read-write. This should prevent the resulting signals that kick off other parts of the boot from happening.
There are no guarantees here though, some other things will be going on in parallel. However, most of the boot will be delayed waiting for access to the filesystem, and in the stated scenario, that seems like the desired result.
Note that there are a lot of good related ideas in the Upstart cookbook