To better understand the request, is the workflow issue more on the creation side, or the display side?
I can understand that it might be easier to specify "50%" in an inspector box to create elements that are half width than doing the math manually or adjusting if you change the scene size.
From a display side, the flexible layout options effectively allow percentage positioning and scaling. If the pins are disabled, the position will be based on its percentage relative to the container. Width/height also are percentage based (unless all pins are enabled which is margin mode or special scaling behavior modes are set).
Let me know if there's some other case I'm not considering that is integral to your workflow.