That is what I had tried and that didn't work. The reason is that the asset call in the standard_layout.html.twig requires a relative path:

https://github.com/sonata-project/SonataAdminBundle/blob/master/Resources/views/standard_layout.html.twig#L115

The default value does not have a leading slash either as you can see here:

https://github.com/sonata-project/SonataAdminBundle/blob/master/DependencyInjection/Configuration.php#L73