Just a quick idea. How about adding a few virtual server variables for specifying backup file/directory names? For example, __DOMAIN__ could substitute the domain name. You could have __ADMIN_USER__, __ADMIN_GROUP__ and I think __HOME_DIR__ would also be useful.
Here's what local file "backup destination" could look like:
__HOME_DIR__/backups/%a_%e-%b-%y_%I-%M-%p to produce "/home/username/Sun_28-Sep-08_10-07-AM"
Note that I'm assuming the backup doesn't backup the entire contents of the home dir, or this example would cause a problem where the file being written was trying to include itself...
I'm more likely to use it like this though:
/back/__ADMIN_USER___%a_%e-%b-%y_%I-%M-%p to give "/back/username_Sun_28-Sep-08_10-07-AM"
With this example, I only have to create 1 scheduled backup which would backup all my virtual hosts. Right now, I have to create an individual backup for each virtual host if I want the admin username hardcoded in the filenames.