I have finally finished the implementation of the Bahtinov Mask Focus Assistant in KStars/EKOS.
It is currently (April 2020) in review and I hope it will be integrated in the official KStars software soon.
At this moment you can get the latest version of the Bahtinov Mask Focus Assistant from my forked KDE/kstars branch:
Just take the master branch, build it and you have the Bahtinov Mask Focus Assistant in kStars.
Good luck and have fun testing it.
Let me know if you need some adjustments, then I can pick that up.