Hi Bram,
You should explore use of the slide master as a way of doing this. I've included a very quick sample that I think might get you where you want to go. It's in Storyline 360.
I put a few objects on the main slide master, with triggers that get tripped on the basis of variables for the three screens in the sample. (See below.) They sit at the very bottom of any timeline, meaning that objects on the screens can obscure them.
Not sure how much you have dug into Storyline yet, but you can have multiple slide masters, if you need these to be different in different scenes. Also, note that I put the triggers and objects on the main slide master, to which the various layouts belong. You can instead put them on specific layouts.
Hope this helps, but if you need clarification or anything else, please let me know.
