A rough work-around could be something like this:
 act=[[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhite];
 [act setFrame:CGRectMake(14, 5, 20, 20)];
 [act startAnimating];
 rightButt=[[UIBarButtonItem alloc] initWithTitle:@"" style:UIBarButtonItemStyleBordered target:self action:nil];
 self.navigationItem.rightBarButtonItem=rightButt;
 if ([[self.navigationController.navigationBar subviews] count]>=2) {
     //Be careful with the next line, Here you get access to an static index, 
     //Apple could change the structure of the navbar and your index may change in the future.
     [[[self.navigationController.navigationBar subviews] objectAtIndex:2] addSubview:act];    
    }
And you'll get this:

EDIT:
From your comment it seems that you want to add this button inside UIToolbar, not in a UINavigationBar, it's pretty the same:
UIActivityIndicatorView* act=[[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhite];
[act setFrame:CGRectMake(10, 13, 20, 20)];
[act startAnimating];
UIBarButtonItem *rightButt=[[UIBarButtonItem alloc] initWithTitle:@"      " style:UIBarButtonItemStyleBordered target:self action:nil];
UIToolbar *tb=[[UIToolbar alloc] initWithFrame:CGRectMake(0, 0, 320, 44)];
[tb setBarStyle:UIBarStyleBlack];
[tb setItems:[NSArray arrayWithObjects:rightButt, nil]];
if ([[tb subviews] count]>=1) {
    [[[tb subviews] objectAtIndex:1] addSubview:act];      
}
and you'll get this:  
