How to extend ApplicationConfiguration to contain additional client properties?
ApplicationConfiguration isn’t really designed to be extensible in its current form, so the easiest option might be to copy the class to your project (in the package where it is in Vaadin) and modify the copy, which will be first on the classpath when comiling your widgetset.
The downside of this approach is that upgrading Vaadin gets more complicated - keeping a patch of the changes made can help there.
Maybe the class will be made easier to extend in some future version if there is need for that, but I’m not aware of concrete short-term plans for that at the moment.