system dbus and Desktop notifications

[Rolf Fokkens <rolf rolffokkens nl>]

Hi!

For a udev driven backup script (connect the USB the disk, and the 
backup starts) I'd like to have desktop notifications that say when the 
backup starts and when the backup is finished so users can disconnect 
the USB disk.
Google is helpful, but seems to tell me that these notifications work 
via the session dbus, not the system dbus. Apparently a solution is to 
identfy any logged on user, find the related DBUS_SESSION_BUS_ADDRESS 
and use that to notify the user.
Is this the best there is? Is there no way to notify (all uses) about 
system events using the system dbus? Are there any plans to create a 
specification (like https://developer.gnome.org/notification-spec/) for 
system notifications?
Thanks,

Rolf