Found a solution !
Ages ago (before we were relying only on Internet), I was told "If you try searching for some explanation start with man man and dig further from there (yep, I was surrounded by prehistorical nerds)". So an old reflex just came back, I've done an old fashion man blender from my terminal and found the following option :
--python-text <name>
Run the given Python script text block
Since your text editor creates a block called Text by default, you just need to specify this block name ... and it works as expected !
While it shouldn't, the online manual seems, somehow, providing less information than the one available locally !
So, execution of random python code can be executed automatically with the following command :
blender -y yourfile.blend --python-text Text
or
blender --enable-autoexec yourfile.blend --python-text Text
where Text is the name / label or your text block (you can get it from the header of your text editor view)
As pointed by batFINGER and dr. Sybren, another easy way is to modify the name generated by default when you create a new text block, by adding ".py" extension to it !