First of all, mad props go to Jérôme Oufella who started the whole thing and dropped a mail.
In current git master you will find a really rudimentary shell extension. This is how it looks:
There is the current activity and duration and you can stop it and call out the standalone.
To install it simply follow the usual installation steps and then restart the shell by pressing Alt+F2 and then “r” for the command.
It’s work in progress of course.
Oh, and if something breaks you can always remove the extension from /usr/local/gnome-shell/extensions/