Hi,
I’m afraid that is not (yet) possible, but it had been requested before, so it will be implemented in one of the upcoming updates for sure.
If you decide to do something custom for it, then to help you out, use the asp_query_args hook to modify the order on the server side.
I suggest maybe injecting an additional drop-down via the asp_layout_settings_before_first_item hook. Any type of input you put in there is then available on the server side in the $_POST['options'] variable.