Unfortunately this would not be possible. While I might be able to handle a "+1" function for the layer number, choosing it freely would be too complicated. And I have to think of the people who want to select things easily... they should not be forced to decide about something like a layer when they click on an item (many people don't even understand the freestyle collection, that's why I worry about that
I would like to include a button in the small pop up menu (the one to move around or delete the item) to switch through layers. But even this would be impossible for me, because after an item was chosen the layer can not be changed anymore.