I’ve been thinking about this as a feature to add to DBC Backup 2 (my plugin). Currently the plugin will backup the WordPress database for your entire multisite.
The problem I found with wanting to backup a subsite is that it is impossible for the subsite ‘owner’ to restore from that backup.
The plugin could save all the tables in the WordPress SQL like wp_7_options and wp_7_posts but
- the sub-site user can not restore the database (they’d need access to PHPMyAdmin for example) and
- they can’t use the backup to make their own WordPress single unless they can edit the SQL.
So all in all … I think this is a nice to have but not sure if it’s worth trying to develop it.