Rated 4 out of 5 stars

Until vaites implements enaut's suggestion, you can create the following shell script (e.g. at ~/bin/transient-notify-send), and configure to use it in the "Gnome Integration" preferences.

#!/bin/bash
notify-send --hint int:transient:1 "$@"

Don't forget to set the script executable.

This review is for a previous version of the add-on (0.4.2). 

Johannes, can you tell me what distro and version are you using? Thanks.