%post @myself computation finished!
This notification looks very useful when a very long computation is done, thanks for this idea 
Here are some more thoughts regarding this project:
Processing the cell
How about wrapping the message in strings?
e.g.
compute_stuff()
%post @myself "computation finished!"
Cell magic can also take variables, so something like this would also be possible:
compute_stuff()
message = "computation finished!"
%post @myself $message
But then, the last line would also be posted.
This could be avoided by having a parameter NOT in the comments, that mutes certain lines from the stream (implementation can be done like here)
compute_stuff()
message = "computation finished!" # NOT
%post @myself $message
Posting to other platforms
It would be interesting to investigate if this posting option can also be possible from a JupyterLab notebook without using JupyterHub.
There are also many other platforms where auto-posting a Jupyter cell and its output can be useful, e.g.
discourse.jupyter, Twitter, Mastodon, Discord, Telegram and gitter.
Do you think that your project would be a good place for support of some of these platforms?
Or would it be more convenient to make separate repos for each platform?
Also, I saw that the project is currently on GitLab.
Therefore, the question: would you consider migrating the project to GitHub?
Advantages would be: it’s easier to contribute for people, and contributions will show up on the GitHub profile, people can star the project, one can use the GitHub CI/CD (e.g. for auto-publishing on pypi), and it’s possible to reference issues and prs in other GitHub projects.