I have tried to change the setApplicationName in the back end by the following process.
I have a text box in admin area which when filled must take and show as the title in the admin area.
For example in LeftAndMain.ss we have
<title>$title</title>

So when the text box is filled with something $title must take the text box value.
For that what i did is I made a CustomLeftAndMian.php

in that i have written a function

public function CustomTitle(){
$titlefirstpart = CustomLeftAndMain::SuperAdminVals()->Providedby; // this returns the text box value
if(!$titlefirstpart)
$titlefirstpart = "Hubflx";
return ($section = $this->owner->SectionTitle()) ? sprintf('%s - %s', $titlefirstpart, $section) : $app;
}

and I have also overridden LeftAndMain.ss and inside that i have
<title>$CustomTitle</title> ..

Now I get the result as expected, that is i get the value from the text box.

But the problem is on ajax load the value changes and takes what is given in mysite/_config.php in LeftAndMain::setApplicationName('Hubflx');

Again when i refresh the page it takes the text box value.. It does not work on ajax load..
Any suggestions to fix this will be greatly appreciated.. Thanks a lot..