A layout modifier would be as simple as the one I proposed in my initial
mail. But making core functionality dependent on a contrib module is a
no-go I think.
Doesn't need to be; the current behavior (or alternatives, probably later) is shifted to be the default value of a new field in the Layout record, and contrib modules can then change it. This change should be transparent to all existing modules. If the change is better then that change might be a candidate for migration to the core.