Ok, so it turns out I missed that critical step . I missed the part in your first post when you said "(it does not install phpVirtualBox)".
After I did that, the phpVirtualBox console app came up right away (though on the WebServer port (81), not the vboxwebsrv port (18083)).
I found most of the install steps from here http://lime-technology.com/forum/index.php?topic=10978.315, but it has superfluous steps you've eliminated with your plugin, so I'm putting the steps I got to work here for anyone who stumbles on this:
Starting state: UnRAID v5.0rc11, SimpleFeatures 1.0.5, VirtualBox PlugIn 0.5.2
1) Downloaded the phpVirtualBox package from here: http://code.google.com/p/phpvirtualbox/downloads/list. I used the phpvirtualbox-4.2-4.zip file as it indicated it was compatible with 4.2.x, which is the most current point release.
2) Unzipped to new directory /boot/tmp (able to copy over the network easily), then moved the entire contents of that folder to /tmp/web (default SimpleFeatures WebServer folder).
3) Renamed config.php-example to config.php. Also changed $username and $password variables, as well as $location variable to point to the LAN IP of the box.
4) Accessed the UI using "http://<UNRAID LAN IP>:81" and logged in.
Looks like everything's working now.
Really like the plugin - I've already used it to update to v4.2.10 and it worked flawlessly.
Thanks!