Is there a reason why you want to use Webacula? We may have other solutions that might work just as well if not better. For instance, check out the CrashPlan app or you can use Clonezilla if you need bare metal backup capability.
http://www.amahi.org/apps/crashplan
There are many free 3rd party backup programs that you can use on most clients to automatically back up to the HDA. I use Cobian Backup on my machines (Windows) and it works quite well for me.
We have been experimenting with BackupPC, but not sure if it is working correctly. The web app appears to install fine, but has not been thoroughly tested.