After some research, I found a very very old thread asking for the same and the bottom line was that this is impossible to achieve.
Since that thread was pretty old, I was wondering if any newly-come third party plugin could help achieving this.
Or...
Is there a music player that can be controlled by Tasker:
* Read a specific file that Tasker will pass in parameters (potentially though an intent)
* Can be paused/resumed by Tasker
AND
* Is able to play the said file when screen is locked and off. (Note: that last point Tasker CAN do).
I hope someone can help.
Cheers