How can I achieve a good socarrat with paella cooked in the oven?
Putting the pan on the burner will deliver too much heat in too little time - aka a burnt layer.

You want constant heat from the bottom during the whole cooking time. In an oven, your pizza stone is an excellent start, supplying a thermal mass. I recommend increasing the heat from the bottom compared to the heat from the top, even if this could slightly increase the total cooking time:
Set your oven to "bottom heat" and / or put the pan on the lowest rack possible. A sheet of foil can also shield the pan from top heat.