Broadcasting a message to users in desktop environments

I'm currently writing a small systemd service that may need to shutdown the computer. I'd like to broadcast a message to all users, e.g. This computer is going to shutdown at XXX, don't forget save your work. For terminal users, I could use wall, but for users who don't have a terminal open, that's not an option. I know that all the users on this machine use X/Gnome, so they have a freedesktop notification client installed. However, it's not clear to me how I can use the system bus (?) to send freedesktop notifications. How can I do that?