From now on it is possible to make a script show a Create new overlay after the script is executed.

This can be done by executing the following call in your script:
ScriptContext.ShowCNI();

By default this will open the Create New with the current list. It is also possible to provide the list for which to create the item:
ScriptContext.ShowCNI(568);

Known issue: when opening two tabs sometimes the cni doesn't get triggered after the first cni on the other tab.