I'd recommend making a new Widget, LoginButtonContainer, which is not @Templated. Use the LoginButtonContainer to dynamically add whatever widget you desire (which could also be @Templated) depending on the login state, and finally use the LoginButtonContainer as a @DataField in your page.
Alright. Sounds interesting, but I am already exploring a rather different approach and it is yielding some profit. Will update this thread soon. Thanks for your help.