Is it possible to create a custom layout as designer plugin?
All the documentation I can find about loading plugins into the Designer is about widgets.
In particular the createWidget() function on the QDesignerCustomWidgetInterface returns a QWidget and given that QLayout does not derive from that class, that method is clearly not possible. But is there another way then?
Good question but I can't answer. I'd recommend brining it on the interest mailing list. You'll find there Qt's developers/maintainers. This forum is more user oriented.