This is possible already.
Here is the post about it:
And the code which helps you to make better experiences! : )
The magic is that, you need to wrap them with a data 
'template' => [ ['acf/my-block', [ 'data' => [ 'field_name' => 'the value you want to see by default', ] ]] ];
I hope it helps you too as well!
I’m stuck with repeater subfields now.
Didn’t find a way to fill them yet.
If any of you able to do that, don’t hesitate to write here about that. 😉
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