Did you have look at the following widget, i think it was made exactly for this :]
What you want to achieve is possible with the form loader widget, but I can't tell you how to get it working :(
I have a project where this works as you want - the formloader is placed on the landing page and after that the data loaded in the header remains there, irrespective of where you navigate to.
I have a second project where the exact same setup produces the results you describe - the data is displayed initially, but cleared when I navigate to another page. I cannot see what the difference is between the 2 projects that causes this difference in behavior. They use the same theme and index page, the same settings on the formloader widget, but they behave differently.
I will keep looking to see if I can identify what makes the difference, and if so will update you here.
Edit: So the difference was that project 2 was using an older version of the widget. I upgraded it to the latest version and it works correctly in both projects now. So Dave, first thing to check is that you have the latest version.
Change index.html. Put a div in the before or the after toolbar. For example
Use this id as shared id in the formloader. Formloader should appear on your firstpage.
See the documentation about theming if you don't know how to change index.html.
The formloader widget get initialized when the page which it's on is requested. When browsing away from that page the widget will be uninitialized. That's why the username will be made empty when browsing away.
Pieter van der Braak mentioned the login button widget? Is that a solution for you Dave? Otherwise you can create your own custom widget to display the username, that's how i do it.
If the formloader has the property 'sharednodeid' set, the form should stay visible until another formloader with the same 'sharednodeid' is loaded. I'm not completely sure if the propery 'cache forms' needs to be enabled as well.
Prerequisite is that the html element with this 'sharednodeid' is placed outside the Mendix content pane in the html file.
If the form does not stay visible this is probably a bug and I suggest filing a bug report.
sharednodeid is not working when changing forms. But we changed the widget to get this working. You can contact me through our company contact form and I'll send this modified (4.x) widget to you.